Release Time Tools 0.1.0

.gitignore

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+node_modules/
+dist/
+release/*.zip
+release/*.zip.sha256
+coverage/
+playwright-report/
+test-results/
+*.tsbuildinfo
+.DS_Store

.npmrc

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+@add-ideas:registry=https://git.add-ideas.de/api/packages/lotobo/npm/

CHANGELOG.md

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+# Changelog
+
+## 0.1.0 - 2026-09-01
+
+- Added the initial local-first Time Tools workbench.
+- Added Toolbox integration, bounded input handling, offline shell and deterministic release packaging.

CONTRIBUTING.md

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+# Contributing
+
+Run `npm ci`, `npm run check`, and `npm run test:browser` before proposing a change. Keep processing local, preserve explicit limits, and include fixtures for every parser boundary.

LICENSE

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+later version.
+
+  15. Disclaimer of Warranty.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.

LICENSES/README.md

@@ -0,0 +1,3 @@
+# Licences
+
+The application is GPL-3.0-or-later. Generated release artifacts include an exact runtime dependency licence inventory in `npm-runtime-licenses.txt`.

README.md

@@ -0,0 +1,33 @@
+# Time Tools
+
+Work with dates, time zones and recurrences locally in the browser.
+
+Time Tools is a standalone local-first application in the [add·ideas Toolbox](https://git.add-ideas.de/lotobo/toolbox-portal). Inputs are processed in the browser and are not uploaded.
+
+## Version 0.1 scope
+
+- Exact signed epoch conversion through nanosecond precision
+- IANA-zone comparison, skipped/ambiguous local-time resolution and transition inspection
+- Side-by-side wall-clock and elapsed-duration arithmetic
+- Bounded 5/6/7-field cron and strict RFC 5545 RRULE occurrence previews
+- Weekend/explicit-holiday business-day arithmetic and escaped UTC iCalendar event export
+
+Named-zone results use the IANA data supplied by the browser runtime and can therefore vary with browser versions. ECMAScript Temporal does not represent leap seconds. The business-day calculator excludes weekends and only the holiday dates supplied by the user; it does not infer regional calendars. See [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) and [docs/PRIVACY-SECURITY.md](docs/PRIVACY-SECURITY.md).
+
+## Development
+
+Requires Node.js 22 and npm 11.
+
+```sh
+npm ci
+npm run check
+npm run test:browser
+```
+
+## Release
+
+`npm run release:artifact` creates a deterministic `release/time-tools-0.1.0.zip` and checksum sidecar.
+
+## Licence
+
+GPL-3.0-or-later. Runtime dependencies retain their own licences; see [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md).

SECURITY.md

@@ -0,0 +1,3 @@
+# Security
+
+Please report vulnerabilities privately to the repository owner. Do not attach sensitive source files to public issues. Imported inputs are untrusted; parsers must remain bounded and must not execute active content or fetch external resources.

SOURCE.md

@@ -0,0 +1,7 @@
+# Corresponding source
+
+The corresponding source for Time Tools 0.1.0 is available at:
+
+https://git.add-ideas.de/lotobo/time-tools/src/tag/v0.1.0
+
+Build with Node.js 22, npm 11, `npm ci`, and `npm run release:artifact`.

THIRD_PARTY_NOTICES.md

@@ -0,0 +1,16 @@
+# Third-party notices
+
+Time Tools 0.1.0 directly depends on these runtime packages:
+
+| Package                          | Pinned version | Declared licence |
+| -------------------------------- | -------------: | ---------------- |
+| `@add-ideas/toolbox-contract`    |          0.2.3 | Apache-2.0       |
+| `@add-ideas/toolbox-helpers`     |          0.1.0 | GPL-3.0-or-later |
+| `@add-ideas/toolbox-shell-react` |          0.2.3 | Apache-2.0       |
+| `croner`                         |         10.0.1 | MIT              |
+| `rrule-temporal`                 |          2.2.2 | MIT              |
+| `temporal-polyfill`              |          1.0.4 | MIT              |
+| `react`                          |         19.2.8 | MIT              |
+| `react-dom`                      |         19.2.8 | MIT              |
+
+This table covers direct production dependencies, not transitive packages or development tooling. During a release build, the exact non-development dependency tree and discovered licence texts are generated from the lockfile into `LICENSES/npm-runtime-licenses.txt` and included in the release ZIP. Copyright and licence terms remain with their respective authors.

docs/ACCESSIBILITY.md

@@ -0,0 +1,3 @@
+# Accessibility
+
+The workbench uses semantic landmarks, headings and labelled native controls, supports keyboard operation, visible focus, reduced-width layouts, and system/light/dark themes through the Toolbox shell. Results do not rely on colour alone.

docs/ARCHITECTURE.md

@@ -0,0 +1,7 @@
+# Architecture
+
+Time Tools is a static React/Vite application wrapped in the shared Toolbox shell. Its pure modules separate exact epoch parsing, named-zone resolution/transition inspection, calendar-versus-elapsed arithmetic, bounded recurrence preview, business-day calculation and UTC iCalendar generation.
+
+`temporal-polyfill` supplies Temporal semantics while the browser runtime supplies IANA time-zone data. Croner handles explicitly selected 5/6/7-field cron syntax; `rrule-temporal` parses strict RFC 5545 recurrence rules with bounded iterations/candidate evaluations. iCalendar export resolves one named-zone wall time and writes escaped/folded UTC DTSTART/DTEND fields rather than an incomplete `VTIMEZONE`.
+
+Version 0.1 operations are synchronous and bounded, so the app creates no worker and has no persistence or server API. Relative entry and asset URLs keep the build relocatable below a nested portal path.

docs/PRIVACY-SECURITY.md

@@ -0,0 +1,7 @@
+# Privacy and security
+
+Time values, schedules and calendar fields stay in page memory. There is no telemetry, analytics, account, persistence or runtime network path. Exported ICS text is escaped and UTF-8 line-folded; users choose whether to download/import it into another application.
+
+Epoch input uses signed integer arithmetic and rejects exponent/fraction syntax. Recurrence output is limited to 1,000 occurrences, RRULE evaluation uses hard iteration/candidate bounds, business-day movement is limited to ±100,000 days, and explicit holidays to 10,000 bounded ISO dates. Calendar text fields and zone/duration inputs are length-bounded before parsing.
+
+Named-zone results depend on the IANA data supplied by the browser/runtime and can vary across versions. Temporal does not represent leap seconds. Cron semantics vary by implementation; the selected field count and common Unix day-of-month/day-of-week OR rule are shown. Business days mean Monday–Friday minus only the dates entered, not a regional legal calendar.

eslint.config.mjs

@@ -0,0 +1,37 @@
+import js from "@eslint/js";
+import globals from "globals";
+import reactHooks from "eslint-plugin-react-hooks";
+import reactRefresh from "eslint-plugin-react-refresh";
+import tseslint from "typescript-eslint";
+
+export default tseslint.config(
+  {
+    ignores: [
+      "dist",
+      "release",
+      "coverage",
+      "test-results",
+      "playwright-report",
+    ],
+  },
+  {
+    extends: [js.configs.recommended, ...tseslint.configs.recommended],
+    files: ["**/*.{ts,tsx}"],
+    languageOptions: {
+      ecmaVersion: 2023,
+      globals: { ...globals.browser, ...globals.worker },
+    },
+    plugins: { "react-hooks": reactHooks, "react-refresh": reactRefresh },
+    rules: {
+      ...reactHooks.configs.recommended.rules,
+      "react-refresh/only-export-components": [
+        "warn",
+        { allowConstantExport: true },
+      ],
+    },
+  },
+  {
+    files: ["scripts/**/*.mjs", "playwright.config.ts"],
+    languageOptions: { globals: globals.node },
+  },
+);

index.html

@@ -0,0 +1,19 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta
+      name="description"
+      content="Work with dates, time zones and recurrences locally in the browser."
+    />
+    <link rel="icon" type="image/svg+xml" href="./favicon.svg" />
+    <link rel="manifest" href="./manifest.webmanifest" />
+    <meta name="theme-color" content="#29255f" />
+    <title>Time Tools</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="./src/main.tsx"></script>
+  </body>
+</html>

package.json

@@ -0,0 +1,74 @@
+{
+  "name": "time-tools",
+  "version": "0.1.0",
+  "description": "Work with dates, time zones and recurrences locally in the browser.",
+  "license": "GPL-3.0-or-later",
+  "author": "Albrecht Degering",
+  "repository": {
+    "type": "git",
+    "url": "git+https://git.add-ideas.de/lotobo/time-tools.git"
+  },
+  "homepage": "https://git.add-ideas.de/lotobo/time-tools",
+  "bugs": {
+    "url": "https://git.add-ideas.de/lotobo/time-tools/issues"
+  },
+  "private": true,
+  "type": "module",
+  "engines": {
+    "node": ">=22"
+  },
+  "scripts": {
+    "predev": "npm run manifest:generate",
+    "dev": "vite",
+    "prebuild": "npm run release:prepare && npm run manifest:generate",
+    "build": "tsc -b && vite build",
+    "preview": "vite preview",
+    "typecheck": "tsc -b --pretty false",
+    "lint": "eslint . --max-warnings=0",
+    "format": "prettier --write .",
+    "format:check": "prettier --check .",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:browser": "playwright test",
+    "manifest:generate": "node scripts/generate-toolbox-manifest.mjs",
+    "manifest:check": "node scripts/generate-toolbox-manifest.mjs --check",
+    "release:prepare": "node scripts/prepare-release-files.mjs",
+    "toolbox:check": "toolbox-check dist",
+    "package:release": "node scripts/package-release.mjs",
+    "check": "npm run manifest:check && npm run typecheck && npm run lint && npm run format:check && npm run test && npm run build && npm run toolbox:check",
+    "release:artifact": "npm run check && npm run test:browser && npm run package:release -- --force"
+  },
+  "dependencies": {
+    "@add-ideas/toolbox-contract": "0.2.3",
+    "@add-ideas/toolbox-helpers": "0.1.0",
+    "@add-ideas/toolbox-shell-react": "0.2.3",
+    "croner": "10.0.1",
+    "react": "19.2.8",
+    "react-dom": "19.2.8",
+    "rrule-temporal": "2.2.2",
+    "temporal-polyfill": "1.0.4"
+  },
+  "devDependencies": {
+    "@add-ideas/toolbox-testkit": "0.2.3",
+    "@eslint/js": "10.0.1",
+    "@playwright/test": "1.62.1",
+    "@testing-library/jest-dom": "6.9.1",
+    "@testing-library/react": "16.3.2",
+    "@testing-library/user-event": "14.6.1",
+    "@types/node": "25.9.5",
+    "@types/react": "19.2.17",
+    "@types/react-dom": "19.2.5",
+    "@vitejs/plugin-react": "6.0.3",
+    "eslint": "10.7.0",
+    "eslint-plugin-react-hooks": "7.1.1",
+    "eslint-plugin-react-refresh": "0.5.2",
+    "globals": "17.7.0",
+    "jsdom": "29.1.1",
+    "prettier": "3.9.5",
+    "typescript": "6.0.3",
+    "typescript-eslint": "8.64.0",
+    "vite": "8.2.2",
+    "vitest": "4.1.11"
+  },
+  "packageManager": "npm@11.17.0"
+}

playwright.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./tests/browser",
+  fullyParallel: false,
+  workers: 1,
+  timeout: 120_000,
+  expect: { timeout: 10_000 },
+  reporter: [["list"]],
+  use: { baseURL: "http://127.0.0.1:4173", trace: "retain-on-failure" },
+  webServer: {
+    command: "npm run build && node scripts/serve-test.mjs",
+    url: "http://127.0.0.1:4173",
+    reuseExistingServer: !process.env.CI,
+    timeout: 180_000,
+  },
+  projects: [
+    { name: "chromium", use: { ...devices["Desktop Chrome"] } },
+    { name: "firefox", use: { ...devices["Desktop Firefox"] } },
+  ],
+});

public/CHANGELOG.md

@@ -0,0 +1,6 @@
+# Changelog
+
+## 0.1.0 - 2026-09-01
+
+- Added the initial local-first Time Tools workbench.
+- Added Toolbox integration, bounded input handling, offline shell and deterministic release packaging.

public/CONTRIBUTING.md

@@ -0,0 +1,3 @@
+# Contributing
+
+Run `npm ci`, `npm run check`, and `npm run test:browser` before proposing a change. Keep processing local, preserve explicit limits, and include fixtures for every parser boundary.

public/LICENSE

@@ -0,0 +1,674 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
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public/LICENSES/README.md

@@ -0,0 +1,3 @@
+# Licences
+
+The application is GPL-3.0-or-later. Generated release artifacts include an exact runtime dependency licence inventory in `npm-runtime-licenses.txt`.

public/README.md

@@ -0,0 +1,33 @@
+# Time Tools
+
+Work with dates, time zones and recurrences locally in the browser.
+
+Time Tools is a standalone local-first application in the [add·ideas Toolbox](https://git.add-ideas.de/lotobo/toolbox-portal). Inputs are processed in the browser and are not uploaded.
+
+## Version 0.1 scope
+
+- Exact signed epoch conversion through nanosecond precision
+- IANA-zone comparison, skipped/ambiguous local-time resolution and transition inspection
+- Side-by-side wall-clock and elapsed-duration arithmetic
+- Bounded 5/6/7-field cron and strict RFC 5545 RRULE occurrence previews
+- Weekend/explicit-holiday business-day arithmetic and escaped UTC iCalendar event export
+
+Named-zone results use the IANA data supplied by the browser runtime and can therefore vary with browser versions. ECMAScript Temporal does not represent leap seconds. The business-day calculator excludes weekends and only the holiday dates supplied by the user; it does not infer regional calendars. See [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) and [docs/PRIVACY-SECURITY.md](docs/PRIVACY-SECURITY.md).
+
+## Development
+
+Requires Node.js 22 and npm 11.
+
+```sh
+npm ci
+npm run check
+npm run test:browser
+```
+
+## Release
+
+`npm run release:artifact` creates a deterministic `release/time-tools-0.1.0.zip` and checksum sidecar.
+
+## Licence
+
+GPL-3.0-or-later. Runtime dependencies retain their own licences; see [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md).

public/SECURITY.md

@@ -0,0 +1,3 @@
+# Security
+
+Please report vulnerabilities privately to the repository owner. Do not attach sensitive source files to public issues. Imported inputs are untrusted; parsers must remain bounded and must not execute active content or fetch external resources.

public/SOURCE.md

@@ -0,0 +1,7 @@
+# Corresponding source
+
+The corresponding source for Time Tools 0.1.0 is available at:
+
+https://git.add-ideas.de/lotobo/time-tools/src/tag/v0.1.0
+
+Build with Node.js 22, npm 11, `npm ci`, and `npm run release:artifact`.

public/THIRD_PARTY_NOTICES.md

@@ -0,0 +1,16 @@
+# Third-party notices
+
+Time Tools 0.1.0 directly depends on these runtime packages:
+
+| Package                          | Pinned version | Declared licence |
+| -------------------------------- | -------------: | ---------------- |
+| `@add-ideas/toolbox-contract`    |          0.2.3 | Apache-2.0       |
+| `@add-ideas/toolbox-helpers`     |          0.1.0 | GPL-3.0-or-later |
+| `@add-ideas/toolbox-shell-react` |          0.2.3 | Apache-2.0       |
+| `croner`                         |         10.0.1 | MIT              |
+| `rrule-temporal`                 |          2.2.2 | MIT              |
+| `temporal-polyfill`              |          1.0.4 | MIT              |
+| `react`                          |         19.2.8 | MIT              |
+| `react-dom`                      |         19.2.8 | MIT              |
+
+This table covers direct production dependencies, not transitive packages or development tooling. During a release build, the exact non-development dependency tree and discovered licence texts are generated from the lockfile into `LICENSES/npm-runtime-licenses.txt` and included in the release ZIP. Copyright and licence terms remain with their respective authors.

public/docs/ACCESSIBILITY.md

@@ -0,0 +1,3 @@
+# Accessibility
+
+The workbench uses semantic landmarks, headings and labelled native controls, supports keyboard operation, visible focus, reduced-width layouts, and system/light/dark themes through the Toolbox shell. Results do not rely on colour alone.

public/docs/ARCHITECTURE.md

@@ -0,0 +1,7 @@
+# Architecture
+
+Time Tools is a static React/Vite application wrapped in the shared Toolbox shell. Its pure modules separate exact epoch parsing, named-zone resolution/transition inspection, calendar-versus-elapsed arithmetic, bounded recurrence preview, business-day calculation and UTC iCalendar generation.
+
+`temporal-polyfill` supplies Temporal semantics while the browser runtime supplies IANA time-zone data. Croner handles explicitly selected 5/6/7-field cron syntax; `rrule-temporal` parses strict RFC 5545 recurrence rules with bounded iterations/candidate evaluations. iCalendar export resolves one named-zone wall time and writes escaped/folded UTC DTSTART/DTEND fields rather than an incomplete `VTIMEZONE`.
+
+Version 0.1 operations are synchronous and bounded, so the app creates no worker and has no persistence or server API. Relative entry and asset URLs keep the build relocatable below a nested portal path.

public/docs/PRIVACY-SECURITY.md

@@ -0,0 +1,7 @@
+# Privacy and security
+
+Time values, schedules and calendar fields stay in page memory. There is no telemetry, analytics, account, persistence or runtime network path. Exported ICS text is escaped and UTF-8 line-folded; users choose whether to download/import it into another application.
+
+Epoch input uses signed integer arithmetic and rejects exponent/fraction syntax. Recurrence output is limited to 1,000 occurrences, RRULE evaluation uses hard iteration/candidate bounds, business-day movement is limited to ±100,000 days, and explicit holidays to 10,000 bounded ISO dates. Calendar text fields and zone/duration inputs are length-bounded before parsing.
+
+Named-zone results depend on the IANA data supplied by the browser/runtime and can vary across versions. Temporal does not represent leap seconds. Cron semantics vary by implementation; the selected field count and common Unix day-of-month/day-of-week OR rule are shown. Business days mean Monday–Friday minus only the dates entered, not a regional legal calendar.

public/favicon.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" role="img" aria-label="Time Tools"><path fill="#5b4ec4" d="M11 4h30l12 12v44H11z"/><path fill="#fff" opacity=".28" d="M41 4v14h12"/><text x="32" y="43" fill="#fff" font-family="system-ui,sans-serif" font-size="18" font-weight="800" text-anchor="middle">TI</text></svg>

public/manifest.webmanifest

@@ -0,0 +1,18 @@
+{
+  "name": "Time Tools",
+  "short_name": "Time",
+  "description": "Work with dates, time zones and recurrences locally in the browser.",
+  "start_url": "./",
+  "scope": "./",
+  "display": "standalone",
+  "background_color": "#f6f7fb",
+  "theme_color": "#29255f",
+  "icons": [
+    {
+      "src": "./favicon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}

public/sw.js

@@ -0,0 +1,54 @@
+const CACHE_PREFIX = "time-tools-shell-";
+const CACHE_NAME = CACHE_PREFIX + "0.1.0";
+const CORE = ["./", "./manifest.webmanifest", "./favicon.svg"];
+self.addEventListener("install", (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then(async (cache) => {
+      await cache.addAll(CORE);
+      const html = await (await fetch("./")).text();
+      const assets = [...html.matchAll(/(?:src|href)="(\.\/assets\/[^"]+)"/g)]
+        .map((match) => match[1])
+        .filter(Boolean);
+      await cache.addAll(assets);
+    }),
+  );
+  self.skipWaiting();
+});
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches
+      .keys()
+      .then((keys) =>
+        Promise.all(
+          keys
+            .filter((key) => key.startsWith(CACHE_PREFIX) && key !== CACHE_NAME)
+            .map((key) => caches.delete(key)),
+        ),
+      )
+      .then(() => self.clients.claim()),
+  );
+});
+self.addEventListener("fetch", (event) => {
+  if (event.request.method !== "GET") return;
+  const url = new URL(event.request.url);
+  if (
+    url.origin !== self.location.origin ||
+    !url.pathname.startsWith(new URL(self.registration.scope).pathname)
+  )
+    return;
+  event.respondWith(
+    caches.match(event.request).then(
+      (cached) =>
+        cached ??
+        fetch(event.request).then((response) => {
+          if (response.ok) {
+            const copy = response.clone();
+            void caches
+              .open(CACHE_NAME)
+              .then((cache) => cache.put(event.request, copy));
+          }
+          return response;
+        }),
+    ),
+  );
+});

public/toolbox-app.json

@@ -0,0 +1,41 @@
+{
+  "$schema": "https://git.add-ideas.de/lotobo/toolbox-sdk/raw/branch/main/schemas/toolbox-app.v1.schema.json",
+  "schemaVersion": 1,
+  "id": "de.add-ideas.time-tools",
+  "name": "Time Tools",
+  "version": "0.1.0",
+  "description": "Work with dates, time zones and recurrences locally in the browser.",
+  "entry": "./",
+  "icon": "./favicon.svg",
+  "categories": ["time", "developer", "productivity"],
+  "tags": ["timestamp", "timezone", "cron", "rrule", "ics"],
+  "integration": {
+    "contextVersion": 1,
+    "launchModes": ["navigate", "new-tab"],
+    "embedding": "unsupported"
+  },
+  "requirements": {
+    "secureContext": false,
+    "workers": false,
+    "indexedDb": false,
+    "crossOriginIsolated": false,
+    "topLevelContext": false
+  },
+  "privacy": {
+    "processing": "local",
+    "fileUploads": false,
+    "telemetry": false,
+    "label": "Inputs stay in this browser; nothing is uploaded."
+  },
+  "source": {
+    "repository": "https://git.add-ideas.de/lotobo/time-tools",
+    "license": "GPL-3.0-or-later"
+  },
+  "actions": [
+    {
+      "id": "source",
+      "label": "Source",
+      "url": "https://git.add-ideas.de/lotobo/time-tools"
+    }
+  ]
+}

scripts/generate-toolbox-manifest.mjs

@@ -0,0 +1,33 @@
+import { readFile, writeFile } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { format } from "prettier";
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const sourcePath = path.join(root, "src/toolbox/manifest.source.json");
+const outputPath = path.join(root, "public/toolbox-app.json");
+const source = JSON.parse(await readFile(sourcePath, "utf8"));
+const pkg = JSON.parse(await readFile(path.join(root, "package.json"), "utf8"));
+const versionSource = await readFile(path.join(root, "src/version.ts"), "utf8");
+const appVersion = /^export const APP_VERSION = "([^"]+)";$/mu.exec(
+  versionSource,
+)?.[1];
+const repository = "https://git.add-ideas.de/lotobo/" + pkg.name;
+if (source.version !== pkg.version || appVersion !== pkg.version)
+  throw new Error("Version identity drift");
+if (
+  source.id !== "de.add-ideas." + pkg.name ||
+  source.source?.repository !== repository ||
+  source.source?.license !== "GPL-3.0-or-later"
+)
+  throw new Error("Manifest source identity is invalid");
+const serialized = await format(JSON.stringify(source), {
+  filepath: outputPath,
+});
+if (process.argv.includes("--check")) {
+  if ((await readFile(outputPath, "utf8").catch(() => "")) !== serialized)
+    throw new Error("public/toolbox-app.json is stale");
+  console.log("Toolbox manifest is synchronized");
+} else {
+  await writeFile(outputPath, serialized);
+  console.log("Generated public/toolbox-app.json");
+}

scripts/package-release.mjs

@@ -0,0 +1,165 @@
+#!/usr/bin/env node
+import { createHash } from "node:crypto";
+import { execFile } from "node:child_process";
+import {
+  access,
+  chmod,
+  copyFile,
+  cp,
+  lstat,
+  mkdir,
+  mkdtemp,
+  readFile,
+  readdir,
+  rename,
+  rm,
+  utimes,
+  writeFile,
+} from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { promisify } from "node:util";
+import { fileURLToPath } from "node:url";
+const execute = promisify(execFile);
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const pkg = JSON.parse(await readFile(path.join(root, "package.json"), "utf8"));
+const argument = (name, fallback) => {
+  const index = process.argv.indexOf(name);
+  return index >= 0 ? process.argv[index + 1] : fallback;
+};
+const output = path.resolve(
+  root,
+  argument("--output", "release/" + pkg.name + "-" + pkg.version + ".zip"),
+);
+const checksumOutput = output + ".sha256";
+const force = process.argv.includes("--force");
+if (
+  path.extname(output).toLowerCase() !== ".zip" ||
+  output === root ||
+  output === path.parse(output).root
+)
+  throw new Error("Unsafe release target");
+const exists = (file) =>
+  access(file).then(
+    () => true,
+    () => false,
+  );
+if (!force && ((await exists(output)) || (await exists(checksumOutput))))
+  throw new Error("Release output exists");
+const input = path.join(root, "dist");
+for (const name of [
+  "index.html",
+  "manifest.webmanifest",
+  "sw.js",
+  "toolbox-app.json",
+  "favicon.svg",
+  "README.md",
+  "CHANGELOG.md",
+  "CONTRIBUTING.md",
+  "LICENSE",
+  "SECURITY.md",
+  "SOURCE.md",
+  "THIRD_PARTY_NOTICES.md",
+  "LICENSES/README.md",
+  "LICENSES/npm-runtime-licenses.txt",
+  "docs/ACCESSIBILITY.md",
+  "docs/ARCHITECTURE.md",
+  "docs/PRIVACY-SECURITY.md",
+]) {
+  const details = await lstat(path.join(input, name)).catch(() => null);
+  if (!details?.isFile() || details.isSymbolicLink())
+    throw new Error("Missing release file: " + name);
+}
+const manifest = JSON.parse(
+  await readFile(path.join(input, "toolbox-app.json"), "utf8"),
+);
+const repository = "https://git.add-ideas.de/lotobo/" + pkg.name;
+if (
+  manifest.id !== "de.add-ideas." + pkg.name ||
+  manifest.version !== pkg.version ||
+  manifest.entry !== "./" ||
+  manifest.icon !== "./favicon.svg" ||
+  manifest.source?.repository !== repository
+)
+  throw new Error("Packaged manifest identity is invalid");
+if (
+  /\b(?:src|href)=["']\//iu.test(
+    await readFile(path.join(input, "index.html"), "utf8"),
+  )
+)
+  throw new Error("Root-absolute asset reference");
+async function collect(directory, prefix = "") {
+  const files = [];
+  for (const entry of (await readdir(directory, { withFileTypes: true })).sort(
+    (a, b) => a.name.localeCompare(b.name),
+  )) {
+    const absolute = path.join(directory, entry.name);
+    const relative = prefix ? prefix + "/" + entry.name : entry.name;
+    if (entry.isSymbolicLink())
+      throw new Error("Symlink in release: " + relative);
+    if (entry.isDirectory()) files.push(...(await collect(absolute, relative)));
+    else if (entry.isFile()) files.push({ absolute, relative });
+    else throw new Error("Unsupported release entry: " + relative);
+  }
+  return files;
+}
+const sourceFiles = await collect(input);
+for (const file of sourceFiles)
+  if (
+    file.relative.endsWith(".map") ||
+    /(?:^|\/)(?:\.env(?:\.|$)|id_rsa|id_ed25519|.*\.pem$|.*\.key$)/iu.test(
+      file.relative,
+    ) ||
+    file.relative.split("/").includes("..")
+  )
+    throw new Error("Forbidden release entry: " + file.relative);
+await mkdir(path.dirname(output), { recursive: true });
+const stagingRoot = await mkdtemp(
+  path.join(os.tmpdir(), pkg.name + "-release-"),
+);
+const publicationRoot = await mkdtemp(
+  path.join(path.dirname(output), "." + pkg.name + "-publish-"),
+);
+const stagedTree = path.join(stagingRoot, "tree");
+const stagedArchive = path.join(stagingRoot, path.basename(output));
+try {
+  await cp(input, stagedTree, { recursive: true });
+  const timestamp = new Date("1980-01-01T00:00:00.000Z");
+  for (const file of await collect(stagedTree)) {
+    await chmod(file.absolute, 0o644);
+    await utimes(file.absolute, timestamp, timestamp);
+  }
+  await execute(
+    "zip",
+    [
+      "-X",
+      "-q",
+      "-9",
+      stagedArchive,
+      ...sourceFiles.map((file) => file.relative),
+    ],
+    {
+      cwd: stagedTree,
+      env: { ...process.env, TZ: "UTC" },
+      maxBuffer: 1024 * 1024,
+    },
+  );
+  const archive = await readFile(stagedArchive);
+  const digest = createHash("sha256").update(archive).digest("hex");
+  const stagedChecksum = stagedArchive + ".sha256";
+  await writeFile(stagedChecksum, digest + "  " + path.basename(output) + "\n");
+  const publicationArchive = path.join(publicationRoot, path.basename(output));
+  const publicationChecksum = publicationArchive + ".sha256";
+  await copyFile(stagedArchive, publicationArchive);
+  await copyFile(stagedChecksum, publicationChecksum);
+  if (force) {
+    await rm(output, { force: true });
+    await rm(checksumOutput, { force: true });
+  }
+  await rename(publicationArchive, output);
+  await rename(publicationChecksum, checksumOutput);
+  console.log("Created " + path.relative(root, output) + "\nSHA-256 " + digest);
+} finally {
+  await rm(stagingRoot, { recursive: true, force: true });
+  await rm(publicationRoot, { recursive: true, force: true });
+}

scripts/prepare-release-files.mjs

@@ -0,0 +1,70 @@
+import { cp, mkdir, readFile, readdir, rm, writeFile } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const destination = path.join(root, "public");
+for (const name of [
+  "LICENSE",
+  "README.md",
+  "CHANGELOG.md",
+  "CONTRIBUTING.md",
+  "SECURITY.md",
+  "SOURCE.md",
+  "THIRD_PARTY_NOTICES.md",
+]) {
+  await readFile(path.join(root, name));
+  await cp(path.join(root, name), path.join(destination, name));
+}
+for (const directory of ["LICENSES", "docs"]) {
+  const output = path.join(destination, directory);
+  await rm(output, { recursive: true, force: true });
+  await cp(path.join(root, directory), output, { recursive: true });
+}
+const lock = JSON.parse(
+  await readFile(path.join(root, "package-lock.json"), "utf8"),
+);
+const sections = [];
+for (const [location, locked] of Object.entries(lock.packages ?? {}).sort(
+  ([a], [b]) => a.localeCompare(b),
+)) {
+  if (!location.includes("node_modules/") || locked.dev === true) continue;
+  const packageDirectory = path.join(root, location);
+  const details = JSON.parse(
+    await readFile(path.join(packageDirectory, "package.json"), "utf8"),
+  );
+  const candidates = (await readdir(packageDirectory))
+    .filter((name) => /^(?:licen[cs]e|copying|notice)(?:\.|$)/iu.test(name))
+    .sort();
+  const texts = [];
+  for (const candidate of candidates) {
+    try {
+      texts.push(
+        "--- " +
+          candidate +
+          " ---\n" +
+          (await readFile(path.join(packageDirectory, candidate), "utf8")),
+      );
+    } catch {
+      /* directory */
+    }
+  }
+  sections.push(
+    "=".repeat(78) +
+      "\n" +
+      details.name +
+      "@" +
+      details.version +
+      "\nDeclared licence: " +
+      (details.license ?? locked.license ?? "See upstream") +
+      "\n" +
+      "=".repeat(78) +
+      "\n" +
+      (texts.join("\n\n") || "See upstream package metadata."),
+  );
+}
+await mkdir(path.join(destination, "LICENSES"), { recursive: true });
+await writeFile(
+  path.join(destination, "LICENSES/npm-runtime-licenses.txt"),
+  sections.join("\n\n").trimEnd() + "\n",
+);
+console.log("Prepared release documentation");

scripts/serve-test.mjs

@@ -0,0 +1,66 @@
+import { createServer } from "node:http";
+import { readFile, stat } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+const root = path.resolve(
+  path.dirname(fileURLToPath(import.meta.url)),
+  "..",
+  "dist",
+);
+const prefix = "/deep/nested/time/";
+const types = new Map([
+  [".css", "text/css; charset=utf-8"],
+  [".html", "text/html; charset=utf-8"],
+  [".js", "text/javascript; charset=utf-8"],
+  [".json", "application/json; charset=utf-8"],
+  [".webmanifest", "application/manifest+json; charset=utf-8"],
+  [".svg", "image/svg+xml"],
+  [".md", "text/markdown; charset=utf-8"],
+  [".txt", "text/plain; charset=utf-8"],
+  [".wasm", "application/wasm"],
+  [".png", "image/png"],
+  [".jpg", "image/jpeg"],
+  [".webp", "image/webp"],
+]);
+const headers = {
+  "Content-Security-Policy":
+    "default-src 'self'; base-uri 'self'; object-src 'none'; frame-ancestors 'none'; form-action 'self'; script-src 'self' 'wasm-unsafe-eval'; style-src 'self' 'unsafe-inline'; img-src 'self' data: blob:; media-src 'self' blob:; connect-src 'self'; worker-src 'self' blob:; manifest-src 'self'",
+  "Cross-Origin-Opener-Policy": "same-origin",
+  "Cross-Origin-Resource-Policy": "same-origin",
+  "Permissions-Policy":
+    "camera=(), microphone=(), geolocation=(), usb=(), payment=()",
+  "Referrer-Policy": "no-referrer",
+  "X-Content-Type-Options": "nosniff",
+};
+const server = createServer(async (request, response) => {
+  try {
+    const url = new URL(request.url ?? "/", "http://127.0.0.1");
+    const relative = decodeURIComponent(url.pathname).startsWith(prefix)
+      ? decodeURIComponent(url.pathname).slice(prefix.length)
+      : decodeURIComponent(url.pathname).replace(/^\/+/, "");
+    const normalized = path.posix.normalize(relative || "index.html");
+    if (
+      normalized === ".." ||
+      normalized.startsWith("../") ||
+      path.isAbsolute(normalized)
+    ) {
+      response.writeHead(400).end("Bad request");
+      return;
+    }
+    let file = path.join(root, normalized);
+    if ((await stat(file).catch(() => null))?.isDirectory())
+      file = path.join(file, "index.html");
+    const content = await readFile(file);
+    response.writeHead(200, {
+      "Content-Type":
+        types.get(path.extname(file)) ?? "application/octet-stream",
+      "Cache-Control": "no-cache",
+      ...headers,
+    });
+    response.end(content);
+  } catch {
+    response.writeHead(404, { "Content-Type": "text/plain; charset=utf-8" });
+    response.end("Not found");
+  }
+});
+server.listen(4173, "127.0.0.1", () => console.log("Test server ready"));

src/App.tsx

@@ -0,0 +1,35 @@
+import { lazy, Suspense, useState } from "react";
+import { AppShell } from "@add-ideas/toolbox-shell-react";
+import "@add-ideas/toolbox-shell-react/styles.css";
+import "./styles.css";
+import { ErrorBoundary } from "./components/ErrorBoundary";
+import { HelpDialog } from "./components/HelpDialog";
+import { manifest } from "./toolbox/manifest";
+
+const Workbench = lazy(async () => ({
+  default: (await import("./components/Workbench")).Workbench,
+}));
+
+export function App() {
+  const [helpOpen, setHelpOpen] = useState(false);
+  return (
+    <ErrorBoundary>
+      <AppShell
+        app={manifest}
+        manifestUrl="./toolbox-app.json"
+        helpAction={{ onClick: () => setHelpOpen(true) }}
+      >
+        <Suspense
+          fallback={
+            <p className="loading" role="status">
+              Preparing Time Tools…
+            </p>
+          }
+        >
+          <Workbench />
+        </Suspense>
+      </AppShell>
+      <HelpDialog open={helpOpen} onClose={() => setHelpOpen(false)} />
+    </ErrorBoundary>
+  );
+}

src/components/ErrorBoundary.tsx

@@ -0,0 +1,27 @@
+import { Component, type ErrorInfo, type ReactNode } from "react";
+
+export class ErrorBoundary extends Component<
+  { children: ReactNode },
+  { error?: Error }
+> {
+  state: { error?: Error } = {};
+  static getDerivedStateFromError(error: Error) {
+    return { error };
+  }
+  componentDidCatch(error: Error, info: ErrorInfo) {
+    console.error("Application failure", error, info);
+  }
+  render() {
+    if (this.state.error)
+      return (
+        <main className="fatal">
+          <h1>Time Tools could not continue</h1>
+          <p>{this.state.error.message}</p>
+          <button type="button" onClick={() => location.reload()}>
+            Reload
+          </button>
+        </main>
+      );
+    return this.props.children;
+  }
+}

src/components/HelpDialog.tsx

@@ -0,0 +1,41 @@
+import { useEffect, useRef } from "react";
+
+export function HelpDialog({
+  open,
+  onClose,
+}: {
+  open: boolean;
+  onClose: () => void;
+}) {
+  const dialog = useRef<HTMLDialogElement>(null);
+  useEffect(() => {
+    const node = dialog.current;
+    if (!node) return;
+    if (open && !node.open) node.showModal();
+    if (!open && node.open) node.close();
+  }, [open]);
+  return (
+    <dialog
+      ref={dialog}
+      className="help-dialog"
+      onClose={onClose}
+      onCancel={onClose}
+      aria-labelledby="help-title"
+    >
+      <div className="dialog-heading">
+        <div>
+          <p className="eyebrow">Local-first help</p>
+          <h2 id="help-title">About Time Tools</h2>
+        </div>
+        <button type="button" onClick={onClose} aria-label="Close help">
+          ×
+        </button>
+      </div>
+      <p>Work with dates, time zones and recurrences locally in the browser.</p>
+      <p>
+        All processing is performed in this browser. Imported data is treated as
+        untrusted and bounded before parsing.
+      </p>
+    </dialog>
+  );
+}

src/components/Workbench.tsx

@@ -0,0 +1,665 @@
+import { useMemo, useState } from "react";
+import { triggerBlobDownload } from "@add-ideas/toolbox-helpers";
+import type { CronMode } from "croner";
+import { Temporal } from "temporal-polyfill";
+import { addBusinessDays, compareArithmetic } from "../time/arithmetic";
+import { inspectEpoch, type EpochUnit } from "../time/epoch";
+import { createUtcEvent } from "../time/ics";
+import { previewCron, previewRRule } from "../time/recurrence";
+import {
+  compareTimeZones,
+  nextTransitions,
+  resolveLocalDateTime,
+} from "../time/zones";
+
+type Tab = "epoch" | "zones" | "arithmetic" | "recurrence" | "business" | "ics";
+interface Output {
+  title: string;
+  text: string;
+  note?: string;
+  download?: { filename: string; type: string; content: string };
+}
+
+const tabs: ReadonlyArray<readonly [Tab, string]> = [
+  ["epoch", "Epoch & ISO"],
+  ["zones", "Time zones & DST"],
+  ["arithmetic", "Arithmetic"],
+  ["recurrence", "Cron & RRULE"],
+  ["business", "Business days"],
+  ["ics", "ICS event"],
+];
+
+function initialInstant(): string {
+  return Temporal.Now.instant().toString({ smallestUnit: "second" });
+}
+function initialLocal(): string {
+  return Temporal.Now.zonedDateTimeISO("Europe/Berlin")
+    .toPlainDateTime()
+    .toString({ smallestUnit: "minute" });
+}
+
+function Result({ output }: { output: Output | undefined }) {
+  if (!output)
+    return (
+      <section className="empty" aria-live="polite">
+        <p>Run a calculation to see its exact, copyable result.</p>
+      </section>
+    );
+  return (
+    <section className="result" aria-live="polite">
+      <div className="panel-heading">
+        <div>
+          <p className="eyebrow">Last successful result</p>
+          <h2>{output.title}</h2>
+        </div>
+        <div className="actions">
+          <button
+            type="button"
+            onClick={() => void navigator.clipboard.writeText(output.text)}
+          >
+            Copy
+          </button>
+          {output.download && (
+            <button
+              type="button"
+              onClick={() =>
+                triggerBlobDownload(
+                  new Blob([output.download!.content], {
+                    type: output.download!.type,
+                  }),
+                  output.download!.filename,
+                )
+              }
+            >
+              Download
+            </button>
+          )}
+        </div>
+      </div>
+      <pre>{output.text}</pre>
+      {output.note && <p className="notice">{output.note}</p>}
+    </section>
+  );
+}
+
+export function Workbench() {
+  const [tab, setTab] = useState<Tab>("epoch");
+  const [output, setOutput] = useState<Output>();
+  const [error, setError] = useState("");
+  const [epoch, setEpoch] = useState(() =>
+    Math.trunc(Date.now() / 1_000).toString(),
+  );
+  const [epochUnit, setEpochUnit] = useState<EpochUnit>("seconds");
+  const [instant, setInstant] = useState(initialInstant);
+  const [local, setLocal] = useState(initialLocal);
+  const [zone, setZone] = useState("Europe/Berlin");
+  const [zones, setZones] = useState(
+    "UTC\nEurope/Berlin\nAmerica/New_York\nAsia/Tokyo",
+  );
+  const [duration, setDuration] = useState("P1DT2H");
+  const [recurrenceKind, setRecurrenceKind] = useState<"cron" | "rrule">(
+    "cron",
+  );
+  const [cron, setCron] = useState("0 9 * * MON-FRI");
+  const [cronMode, setCronMode] = useState<CronMode>("5-part");
+  const [rrule, setRrule] = useState("FREQ=WEEKLY;COUNT=12;BYDAY=MO,WE,FR");
+  const [occurrenceCount, setOccurrenceCount] = useState(12);
+  const [businessDate, setBusinessDate] = useState(() =>
+    Temporal.Now.plainDateISO().toString(),
+  );
+  const [businessCount, setBusinessCount] = useState(10);
+  const [holidays, setHolidays] = useState("");
+  const [summary, setSummary] = useState("Toolbox event");
+  const [description, setDescription] = useState("");
+  const [location, setLocation] = useState("");
+  const [eventDuration, setEventDuration] = useState("PT1H");
+
+  const zoneList = useMemo(
+    () =>
+      zones.length > 32_768
+        ? undefined
+        : zones
+            .replaceAll("\r\n", "\n")
+            .split("\n")
+            .map((value) => value.trim())
+            .filter(Boolean),
+    [zones],
+  );
+  const run = (
+    title: string,
+    operation: () => {
+      value: unknown;
+      note?: string;
+      download?: Output["download"];
+    },
+  ) => {
+    try {
+      const result = operation();
+      setOutput({
+        title,
+        text:
+          typeof result.value === "string"
+            ? result.value
+            : JSON.stringify(result.value, null, 2),
+        note: result.note,
+        download: result.download,
+      });
+      setError("");
+    } catch (reason) {
+      setError(
+        reason instanceof Error ? reason.message : "Calculation failed.",
+      );
+    }
+  };
+
+  const epochWorkspace = (
+    <>
+      <div className="form-grid">
+        <label className="field">
+          <span>Epoch value</span>
+          <input
+            inputMode="decimal"
+            value={epoch}
+            onChange={(event) => setEpoch(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>Interpret as</span>
+          <select
+            value={epochUnit}
+            onChange={(event) => setEpochUnit(event.target.value as EpochUnit)}
+          >
+            <option value="seconds">Seconds</option>
+            <option value="milliseconds">Milliseconds</option>
+            <option value="microseconds">Microseconds</option>
+            <option value="nanoseconds">Nanoseconds</option>
+          </select>
+        </label>
+      </div>
+      <button
+        className="primary"
+        type="button"
+        onClick={() =>
+          run("Exact timestamp conversion", () => {
+            const result = inspectEpoch(epoch, epochUnit);
+            return {
+              value: {
+                ...result,
+                epochNanoseconds: result.epochNanoseconds.toString(),
+              },
+            };
+          })
+        }
+      >
+        Convert exactly
+      </button>
+      <p className="muted">
+        Integer math preserves negative, pre-1970, microsecond, and nanosecond
+        values. Exponent notation is intentionally rejected.
+      </p>
+    </>
+  );
+
+  const zonesWorkspace = (
+    <>
+      <div className="form-grid">
+        <label className="field">
+          <span>Instant (ISO with offset)</span>
+          <input
+            value={instant}
+            onChange={(event) => setInstant(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>Local wall time</span>
+          <input
+            type="datetime-local"
+            step="1"
+            value={local}
+            onChange={(event) => setLocal(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>IANA time zone</span>
+          <input
+            value={zone}
+            onChange={(event) => setZone(event.target.value)}
+          />
+        </label>
+      </div>
+      <label className="field">
+        <span>Comparison zones (one per line, max 24)</span>
+        <textarea
+          className="short"
+          value={zones}
+          onChange={(event) => setZones(event.target.value)}
+        />
+      </label>
+      <div className="actions">
+        <button
+          className="primary"
+          type="button"
+          onClick={() =>
+            run("Same instant across zones", () => {
+              if (!zoneList)
+                throw new Error(
+                  "Comparison-zone input exceeds 32,768 UTF-16 units.",
+                );
+              return { value: compareTimeZones(instant, zoneList) };
+            })
+          }
+        >
+          Compare zones
+        </button>
+        <button
+          type="button"
+          onClick={() =>
+            run("Wall-time resolution", () => {
+              const value = resolveLocalDateTime(local, zone);
+              return {
+                value,
+                note:
+                  value.status === "ambiguous"
+                    ? "This wall time occurs twice. Choose an offset deliberately."
+                    : value.status === "skipped"
+                      ? "This wall time does not occur. Earlier/later show Temporal's two explicit resolutions around the gap."
+                      : "This wall time maps to one instant.",
+              };
+            })
+          }
+        >
+          Inspect local time
+        </button>
+        <button
+          type="button"
+          onClick={() =>
+            run("Upcoming offset transitions", () => ({
+              value: nextTransitions(instant, zone, 8),
+              note: "Transition data comes from the time-zone database bundled with this browser runtime and can differ between browser versions.",
+            }))
+          }
+        >
+          Show DST transitions
+        </button>
+      </div>
+    </>
+  );
+
+  const arithmeticWorkspace = (
+    <>
+      <div className="form-grid">
+        <label className="field">
+          <span>Start instant</span>
+          <input
+            value={instant}
+            onChange={(event) => setInstant(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>IANA time zone</span>
+          <input
+            value={zone}
+            onChange={(event) => setZone(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>ISO 8601 duration</span>
+          <input
+            value={duration}
+            onChange={(event) => setDuration(event.target.value)}
+            placeholder="P1DT2H"
+          />
+        </label>
+      </div>
+      <button
+        className="primary"
+        type="button"
+        onClick={() =>
+          run("Wall-clock versus elapsed arithmetic", () => ({
+            value: compareArithmetic(instant, zone, duration),
+            note: "Wall-clock addition preserves calendar intent. Elapsed addition treats weeks and days as exact 7×24 and 24-hour spans; years and months have no fixed elapsed length.",
+          }))
+        }
+      >
+        Compare additions
+      </button>
+    </>
+  );
+
+  const recurrenceWorkspace = (
+    <>
+      <div className="segmented" role="group" aria-label="Recurrence syntax">
+        <button
+          type="button"
+          aria-pressed={recurrenceKind === "cron"}
+          onClick={() => setRecurrenceKind("cron")}
+        >
+          Cron
+        </button>
+        <button
+          type="button"
+          aria-pressed={recurrenceKind === "rrule"}
+          onClick={() => setRecurrenceKind("rrule")}
+        >
+          RRULE
+        </button>
+      </div>
+      {recurrenceKind === "cron" ? (
+        <>
+          <div className="form-grid">
+            <label className="field wide">
+              <span>Cron expression</span>
+              <input
+                value={cron}
+                onChange={(event) => setCron(event.target.value)}
+              />
+            </label>
+            <label className="field">
+              <span>Dialect / fields</span>
+              <select
+                value={cronMode}
+                onChange={(event) =>
+                  setCronMode(event.target.value as CronMode)
+                }
+              >
+                <option value="5-part">5 fields</option>
+                <option value="6-part">6 fields</option>
+                <option value="7-part">7 fields</option>
+                <option value="auto">Auto-detect</option>
+              </select>
+            </label>
+            <label className="field">
+              <span>Start instant</span>
+              <input
+                value={instant}
+                onChange={(event) => setInstant(event.target.value)}
+              />
+            </label>
+            <label className="field">
+              <span>Time zone</span>
+              <input
+                value={zone}
+                onChange={(event) => setZone(event.target.value)}
+              />
+            </label>
+          </div>
+          <button
+            className="primary"
+            type="button"
+            onClick={() =>
+              run("Cron occurrence preview", () => ({
+                value: previewCron(
+                  cron,
+                  instant,
+                  zone,
+                  occurrenceCount,
+                  cronMode,
+                ),
+                note: "Day-of-month and day-of-week use common Unix OR semantics. Choose the field count explicitly when portability matters.",
+              }))
+            }
+          >
+            Preview cron
+          </button>
+        </>
+      ) : (
+        <>
+          <label className="field">
+            <span>RFC 5545 RRULE</span>
+            <textarea
+              className="short"
+              value={rrule}
+              onChange={(event) => setRrule(event.target.value)}
+            />
+          </label>
+          <div className="form-grid">
+            <label className="field">
+              <span>DTSTART local wall time</span>
+              <input
+                type="datetime-local"
+                step="1"
+                value={local}
+                onChange={(event) => setLocal(event.target.value)}
+              />
+            </label>
+            <label className="field">
+              <span>Time zone</span>
+              <input
+                value={zone}
+                onChange={(event) => setZone(event.target.value)}
+              />
+            </label>
+          </div>
+          <button
+            className="primary"
+            type="button"
+            onClick={() =>
+              run("RRULE occurrence preview", () => ({
+                value: previewRRule(rrule, local, zone, occurrenceCount),
+                note: "Strict RFC 5545 parsing is used with bounded iteration and candidate limits. Offset changes remain visible in every occurrence.",
+              }))
+            }
+          >
+            Preview RRULE
+          </button>
+        </>
+      )}
+      <label className="field count">
+        <span>Occurrences (max 1,000)</span>
+        <input
+          type="number"
+          min="1"
+          max="1000"
+          value={occurrenceCount}
+          onChange={(event) => setOccurrenceCount(event.target.valueAsNumber)}
+        />
+      </label>
+    </>
+  );
+
+  const businessWorkspace = (
+    <>
+      <div className="form-grid">
+        <label className="field">
+          <span>Start date</span>
+          <input
+            type="date"
+            value={businessDate}
+            onChange={(event) => setBusinessDate(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>Business days to add (negative allowed)</span>
+          <input
+            type="number"
+            min="-100000"
+            max="100000"
+            value={businessCount}
+            onChange={(event) => setBusinessCount(event.target.valueAsNumber)}
+          />
+        </label>
+      </div>
+      <label className="field">
+        <span>Excluded holiday dates, one ISO date per line</span>
+        <textarea
+          className="short"
+          value={holidays}
+          onChange={(event) => setHolidays(event.target.value)}
+          placeholder="2026-12-25"
+        />
+      </label>
+      <button
+        className="primary"
+        type="button"
+        onClick={() =>
+          run("Business-day result", () => {
+            if (holidays.length > 640_000)
+              throw new Error("Holiday input exceeds 640,000 UTF-16 units.");
+            return {
+              value: addBusinessDays(
+                businessDate,
+                businessCount,
+                holidays.split(/\r?\n/u),
+              ),
+              note: "This calculator excludes Saturday, Sunday, and only the holiday dates you enter. It does not infer regional holidays.",
+            };
+          })
+        }
+      >
+        Calculate business date
+      </button>
+    </>
+  );
+
+  const icsWorkspace = (
+    <>
+      <div className="form-grid">
+        <label className="field">
+          <span>Start local wall time</span>
+          <input
+            type="datetime-local"
+            step="1"
+            value={local}
+            onChange={(event) => setLocal(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>Time zone</span>
+          <input
+            value={zone}
+            onChange={(event) => setZone(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>Duration</span>
+          <input
+            value={eventDuration}
+            onChange={(event) => setEventDuration(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>Summary</span>
+          <input
+            value={summary}
+            onChange={(event) => setSummary(event.target.value)}
+          />
+        </label>
+        <label className="field">
+          <span>Location (optional)</span>
+          <input
+            value={location}
+            onChange={(event) => setLocation(event.target.value)}
+          />
+        </label>
+      </div>
+      <label className="field">
+        <span>Description (optional)</span>
+        <textarea
+          className="short"
+          value={description}
+          onChange={(event) => setDescription(event.target.value)}
+        />
+      </label>
+      <button
+        className="primary"
+        type="button"
+        onClick={() =>
+          run("UTC iCalendar event", () => {
+            const created = createUtcEvent({
+              startLocal: local,
+              timeZone: zone,
+              duration: eventDuration,
+              summary,
+              description,
+              location,
+            });
+            return {
+              value: created.ics,
+              note: created.note,
+              download: {
+                filename: "event.ics",
+                type: "text/calendar;charset=utf-8",
+                content: created.ics,
+              },
+            };
+          })
+        }
+      >
+        Create and download ICS
+      </button>
+    </>
+  );
+
+  const workspace =
+    tab === "epoch"
+      ? epochWorkspace
+      : tab === "zones"
+        ? zonesWorkspace
+        : tab === "arithmetic"
+          ? arithmeticWorkspace
+          : tab === "recurrence"
+            ? recurrenceWorkspace
+            : tab === "business"
+              ? businessWorkspace
+              : icsWorkspace;
+  return (
+    <main className="workbench">
+      <header className="hero">
+        <div>
+          <p className="eyebrow">Calendar-aware local workbench</p>
+          <h1>Time Tools</h1>
+          <p>
+            Convert exact timestamps, inspect time-zone edge cases, compare
+            arithmetic, preview recurrences, and create portable calendar
+            events.
+          </p>
+        </div>
+        <span className="privacy-pill">No network requests</span>
+      </header>
+      <nav
+        className="panel workspace-tabs"
+        role="tablist"
+        aria-label="Time workspaces"
+      >
+        {tabs.map(([value, label]) => (
+          <button
+            type="button"
+            role="tab"
+            aria-selected={tab === value}
+            key={value}
+            onClick={() => setTab(value)}
+          >
+            {label}
+          </button>
+        ))}
+      </nav>
+      <div className="workspace-layout">
+        <section
+          className="panel workspace"
+          aria-labelledby="workspace-heading"
+        >
+          <div>
+            <p className="eyebrow">Calculator</p>
+            <h2 id="workspace-heading">
+              {tabs.find(([value]) => value === tab)?.[1]}
+            </h2>
+          </div>
+          {workspace}
+          {error && (
+            <p className="error" role="alert">
+              {error}
+            </p>
+          )}
+        </section>
+        <Result output={output} />
+      </div>
+      <section className="panel">
+        <p className="notice">
+          Named-zone calculations use real offset transitions, not fixed-offset
+          approximations. The browser runtime supplies its own IANA time-zone
+          data; leap seconds are not represented by ECMAScript Temporal.
+        </p>
+      </section>
+    </main>
+  );
+}

src/main.tsx

@@ -0,0 +1,18 @@
+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import { App } from "./App";
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+);
+
+if ("serviceWorker" in navigator && import.meta.env.PROD) {
+  window.addEventListener("load", () => {
+    const url = new URL("./sw.js", document.baseURI);
+    void navigator.serviceWorker
+      .register(url, { scope: new URL("./", document.baseURI).pathname })
+      .catch(() => undefined);
+  });
+}

src/styles.css

@@ -0,0 +1,276 @@
+:root {
+  --toolbox-background: #f6f7fb;
+  --toolbox-surface: #fff;
+  --toolbox-surface-soft: #eff1f7;
+  --toolbox-text: #202332;
+  --toolbox-muted: #656b7d;
+  --toolbox-border: #d9dce7;
+  --toolbox-accent: #5b4ec4;
+  --toolbox-accent-hover: #493caf;
+  --toolbox-accent-soft: #ece9ff;
+  --toolbox-accent-contrast: #fff;
+  --toolbox-focus: #137d75;
+  --toolbox-danger: #b42342;
+}
+* {
+  box-sizing: border-box;
+}
+html {
+  min-width: 20rem;
+  min-height: 100%;
+  background: var(--toolbox-background);
+  scrollbar-gutter: stable;
+}
+body {
+  min-width: 20rem;
+  min-height: 100vh;
+  margin: 0;
+  background: var(--toolbox-background);
+  color: var(--toolbox-text);
+  font-family: Inter, ui-sans-serif, system-ui, sans-serif;
+}
+button,
+input,
+select,
+textarea {
+  font: inherit;
+}
+button {
+  min-height: 2.55rem;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  gap: 0.4rem;
+  padding: 0.55rem 0.8rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.65rem;
+  background: var(--toolbox-surface);
+  color: var(--toolbox-text);
+  font-weight: 720;
+  cursor: pointer;
+}
+button:hover:not(:disabled) {
+  border-color: var(--toolbox-accent);
+  background: var(--toolbox-surface-soft);
+}
+:where(button, input, select, textarea, a):focus-visible {
+  outline: 3px solid color-mix(in srgb, var(--toolbox-focus) 42%, transparent);
+  outline-offset: 2px;
+}
+input,
+select,
+textarea {
+  width: 100%;
+  min-height: 2.55rem;
+  padding: 0.58rem 0.7rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.62rem;
+  background: var(--toolbox-surface);
+  color: var(--toolbox-text);
+}
+textarea {
+  min-height: 9rem;
+  resize: vertical;
+  font-family: ui-monospace, SFMono-Regular, Consolas, monospace;
+  line-height: 1.48;
+}
+textarea.short {
+  min-height: 6rem;
+}
+.toolbox-shell__main {
+  width: min(100%, 90rem);
+  padding: clamp(0.75rem, 1.8vw, 1.5rem);
+}
+.workbench {
+  display: grid;
+  gap: 1rem;
+}
+.hero,
+.panel,
+.result,
+.empty {
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.9rem;
+  background: var(--toolbox-surface);
+  box-shadow: 0 8px 28px rgb(30 36 70 / 4%);
+}
+.hero {
+  display: flex;
+  justify-content: space-between;
+  gap: 1rem;
+  align-items: flex-start;
+  padding: clamp(1.1rem, 3vw, 2rem);
+}
+.hero h1,
+.panel h2,
+.result h2,
+.help-dialog h2,
+.fatal h1 {
+  margin: 0;
+  letter-spacing: -0.025em;
+}
+.hero p:not(.eyebrow) {
+  max-width: 55rem;
+  margin: 0.55rem 0 0;
+  color: var(--toolbox-muted);
+  line-height: 1.55;
+}
+.eyebrow {
+  margin: 0 0 0.3rem;
+  color: var(--toolbox-accent);
+  font-size: 0.69rem;
+  font-weight: 820;
+  letter-spacing: 0.115em;
+  text-transform: uppercase;
+}
+.privacy-pill {
+  flex: 0 0 auto;
+  padding: 0.38rem 0.62rem;
+  border-radius: 999px;
+  background: var(--toolbox-accent-soft);
+  color: var(--toolbox-accent);
+  font-size: 0.75rem;
+  font-weight: 760;
+}
+.panel,
+.result {
+  padding: 1rem;
+}
+.panel-heading {
+  display: flex;
+  justify-content: space-between;
+  gap: 1rem;
+  align-items: end;
+  margin-bottom: 0.9rem;
+}
+.workspace-tabs {
+  display: flex;
+  gap: 0.4rem;
+  overflow-x: auto;
+  padding: 0.75rem;
+}
+.workspace-tabs button[aria-selected="true"],
+.segmented button[aria-pressed="true"] {
+  border-color: var(--toolbox-accent);
+  background: var(--toolbox-accent);
+  color: var(--toolbox-accent-contrast);
+}
+.workspace-layout {
+  display: grid;
+  grid-template-columns: minmax(20rem, 1fr) minmax(22rem, 1.2fr);
+  gap: 1rem;
+  align-items: start;
+}
+.workspace,
+.result {
+  display: grid;
+  gap: 0.9rem;
+  min-width: 0;
+}
+.form-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(min(100%, 12rem), 1fr));
+  gap: 0.65rem;
+}
+.field {
+  display: grid;
+  gap: 0.35rem;
+  min-width: 0;
+}
+.field > span {
+  font-size: 0.76rem;
+  font-weight: 750;
+}
+.field.wide {
+  grid-column: span 2;
+}
+.field.count {
+  max-width: 12rem;
+}
+.actions,
+.segmented {
+  display: flex;
+  gap: 0.55rem;
+  flex-wrap: wrap;
+}
+.primary {
+  border-color: var(--toolbox-accent);
+  background: var(--toolbox-accent);
+  color: var(--toolbox-accent-contrast);
+}
+.muted {
+  margin: 0;
+  color: var(--toolbox-muted);
+  line-height: 1.5;
+}
+.result pre {
+  max-height: 38rem;
+  margin: 0;
+  padding: 0.8rem;
+  overflow: auto;
+  border-radius: 0.65rem;
+  background: var(--toolbox-surface-soft);
+  white-space: pre-wrap;
+  overflow-wrap: anywhere;
+}
+.empty {
+  min-height: 12rem;
+  display: grid;
+  place-items: center;
+  padding: 2rem;
+  color: var(--toolbox-muted);
+  text-align: center;
+}
+.error,
+.notice {
+  margin: 0;
+  padding: 0.7rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.65rem;
+  line-height: 1.5;
+}
+.error {
+  border-color: var(--toolbox-danger);
+  color: var(--toolbox-danger);
+}
+.notice {
+  background: var(--toolbox-surface-soft);
+}
+.loading,
+.fatal {
+  width: min(100% - 2rem, 60rem);
+  margin: 2rem auto;
+  padding: 1rem;
+}
+.help-dialog {
+  width: min(36rem, calc(100% - 2rem));
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.9rem;
+  background: var(--toolbox-surface);
+  color: var(--toolbox-text);
+}
+.help-dialog::backdrop {
+  background: rgb(20 24 45 / 55%);
+}
+.dialog-heading {
+  display: flex;
+  justify-content: space-between;
+  gap: 1rem;
+  align-items: start;
+}
+@media (max-width: 66rem) {
+  .workspace-layout {
+    grid-template-columns: 1fr;
+  }
+}
+@media (max-width: 42rem) {
+  .hero {
+    flex-direction: column;
+  }
+  .privacy-pill {
+    order: -1;
+  }
+  .field.wide {
+    grid-column: auto;
+  }
+}

src/test/setup.ts

@@ -0,0 +1,8 @@
+import "@testing-library/jest-dom/vitest";
+import { afterEach } from "vitest";
+import { cleanup } from "@testing-library/react";
+
+afterEach(() => {
+  cleanup();
+  localStorage.clear();
+});

src/time/arithmetic.ts

@@ -0,0 +1,147 @@
+import { assertBoundedText } from "@add-ideas/toolbox-helpers";
+import { Temporal } from "temporal-polyfill";
+import { parseInstant } from "./epoch";
+
+const NS = {
+  week: 604_800_000_000_000n,
+  day: 86_400_000_000_000n,
+  hour: 3_600_000_000_000n,
+  minute: 60_000_000_000n,
+  second: 1_000_000_000n,
+  millisecond: 1_000_000n,
+  microsecond: 1_000n,
+};
+
+export interface ArithmeticComparison {
+  start: string;
+  duration: string;
+  timeZone: string;
+  wallClockResult: string;
+  wallClockInstant: string;
+  elapsedResult?: string;
+  elapsedInstant?: string;
+  elapsedUnavailable?: string;
+  differenceSeconds?: string;
+}
+
+function elapsedNanoseconds(duration: Temporal.Duration): bigint {
+  if (duration.years || duration.months)
+    throw new RangeError(
+      "Elapsed arithmetic cannot assign a fixed length to years or months.",
+    );
+  const integral = [
+    duration.weeks,
+    duration.days,
+    duration.hours,
+    duration.minutes,
+    duration.seconds,
+    duration.milliseconds,
+    duration.microseconds,
+    duration.nanoseconds,
+  ].every(Number.isSafeInteger);
+  if (!integral)
+    throw new RangeError(
+      "Elapsed arithmetic requires integral duration fields.",
+    );
+  return (
+    BigInt(duration.weeks) * NS.week +
+    BigInt(duration.days) * NS.day +
+    BigInt(duration.hours) * NS.hour +
+    BigInt(duration.minutes) * NS.minute +
+    BigInt(duration.seconds) * NS.second +
+    BigInt(duration.milliseconds) * NS.millisecond +
+    BigInt(duration.microseconds) * NS.microsecond +
+    BigInt(duration.nanoseconds)
+  );
+}
+
+function exactSeconds(nanoseconds: bigint): string {
+  const negative = nanoseconds < 0n;
+  const absolute = negative ? -nanoseconds : nanoseconds;
+  const whole = absolute / NS.second;
+  const remainder = absolute % NS.second;
+  return `${negative ? "-" : ""}${whole}${remainder ? `.${remainder.toString().padStart(9, "0").replace(/0+$/u, "")}` : ""}`;
+}
+
+export function compareArithmetic(
+  startInput: string,
+  timeZoneInput: string,
+  durationInput: string,
+): ArithmeticComparison {
+  const start = parseInstant(startInput);
+  const timeZone = assertBoundedText(timeZoneInput, 128, "Time zone").trim();
+  const duration = Temporal.Duration.from(
+    assertBoundedText(durationInput, 256, "Duration").trim(),
+  );
+  const zonedStart = start.toZonedDateTimeISO(timeZone);
+  const wall = zonedStart.add(duration);
+  const result: ArithmeticComparison = {
+    start: zonedStart.toString(),
+    duration: duration.toString(),
+    timeZone,
+    wallClockResult: wall.toString(),
+    wallClockInstant: wall.toInstant().toString(),
+  };
+  try {
+    const elapsedNs = elapsedNanoseconds(duration);
+    const elapsed = Temporal.Instant.fromEpochNanoseconds(
+      start.epochNanoseconds + elapsedNs,
+    );
+    result.elapsedResult = elapsed.toZonedDateTimeISO(timeZone).toString();
+    result.elapsedInstant = elapsed.toString();
+    result.differenceSeconds = exactSeconds(
+      wall.epochNanoseconds - elapsed.epochNanoseconds,
+    );
+  } catch (reason) {
+    result.elapsedUnavailable =
+      reason instanceof Error
+        ? reason.message
+        : "Elapsed result is unavailable.";
+  }
+  return result;
+}
+
+export function addBusinessDays(
+  startInput: string,
+  countInput: number,
+  holidayInputs: string[] = [],
+): {
+  start: string;
+  result: string;
+  traversedCalendarDays: number;
+  holidays: string[];
+} {
+  if (!Number.isSafeInteger(countInput) || Math.abs(countInput) > 100_000)
+    throw new RangeError("Business-day count must be within ±100,000.");
+  const count = countInput;
+  const start = Temporal.PlainDate.from(
+    assertBoundedText(startInput, 64, "Start date").trim(),
+  );
+  let current = start;
+  if (holidayInputs.length > 10_000)
+    throw new RangeError("Holiday list is limited to 10,000 dates.");
+  const holidays = new Set(
+    holidayInputs
+      .filter(Boolean)
+      .map((value) =>
+        Temporal.PlainDate.from(
+          assertBoundedText(value, 64, "Holiday date").trim(),
+        ).toString(),
+      ),
+  );
+  const direction = count < 0 ? -1 : 1;
+  let remaining = Math.abs(count);
+  let traversedCalendarDays = 0;
+  while (remaining > 0) {
+    current = current.add({ days: direction });
+    traversedCalendarDays += 1;
+    if (current.dayOfWeek < 6 && !holidays.has(current.toString()))
+      remaining -= 1;
+  }
+  return {
+    start: start.toString(),
+    result: current.toString(),
+    traversedCalendarDays,
+    holidays: [...holidays].sort(),
+  };
+}

src/time/epoch.ts

@@ -0,0 +1,86 @@
+import { assertBoundedText } from "@add-ideas/toolbox-helpers";
+import { Temporal } from "temporal-polyfill";
+
+export type EpochUnit =
+  "seconds" | "milliseconds" | "microseconds" | "nanoseconds";
+
+const FACTORS: Record<EpochUnit, bigint> = {
+  seconds: 1_000_000_000n,
+  milliseconds: 1_000_000n,
+  microseconds: 1_000n,
+  nanoseconds: 1n,
+};
+
+export interface EpochInspection {
+  input: string;
+  interpretedAs: EpochUnit;
+  epochNanoseconds: bigint;
+  seconds: string;
+  milliseconds: string;
+  microseconds: string;
+  nanoseconds: string;
+  iso: string;
+}
+
+function decimalToNanoseconds(input: string, unit: EpochUnit): bigint {
+  const value = assertBoundedText(input, 128, "Timestamp").trim();
+  const match = /^([+-]?)(\d+)(?:\.(\d+))?$/u.exec(value);
+  if (!match)
+    throw new SyntaxError(
+      "Use an integer or decimal epoch value without exponent notation.",
+    );
+  const fraction = match[3] ?? "";
+  const denominator = 10n ** BigInt(fraction.length);
+  const factor = FACTORS[unit];
+  const fractionalNumerator = BigInt(fraction || "0") * factor;
+  if (fractionalNumerator % denominator !== 0n) {
+    throw new RangeError(
+      "The value contains precision smaller than one nanosecond.",
+    );
+  }
+  const magnitude =
+    BigInt(match[2] ?? "0") * factor + fractionalNumerator / denominator;
+  return match[1] === "-" ? -magnitude : magnitude;
+}
+
+function scaledInteger(value: bigint, factor: bigint): string {
+  const negative = value < 0n;
+  const absolute = negative ? -value : value;
+  const whole = absolute / factor;
+  const remainder = absolute % factor;
+  if (remainder === 0n) return `${negative ? "-" : ""}${whole}`;
+  const width = factor.toString().length - 1;
+  const fraction = remainder
+    .toString()
+    .padStart(width, "0")
+    .replace(/0+$/u, "");
+  return `${negative ? "-" : ""}${whole}.${fraction}`;
+}
+
+export function inspectEpoch(input: string, unit: EpochUnit): EpochInspection {
+  const epochNanoseconds = decimalToNanoseconds(input, unit);
+  const instant = Temporal.Instant.fromEpochNanoseconds(epochNanoseconds);
+  return {
+    input: input.trim(),
+    interpretedAs: unit,
+    epochNanoseconds,
+    seconds: scaledInteger(epochNanoseconds, FACTORS.seconds),
+    milliseconds: scaledInteger(epochNanoseconds, FACTORS.milliseconds),
+    microseconds: scaledInteger(epochNanoseconds, FACTORS.microseconds),
+    nanoseconds: epochNanoseconds.toString(),
+    iso: instant.toString(),
+  };
+}
+
+export function parseInstant(
+  input: string,
+  numericUnit: EpochUnit = "seconds",
+): Temporal.Instant {
+  const value = assertBoundedText(input, 256, "Instant").trim();
+  if (/^[+-]?\d+(?:\.\d+)?$/u.test(value)) {
+    return Temporal.Instant.fromEpochNanoseconds(
+      decimalToNanoseconds(value, numericUnit),
+    );
+  }
+  return Temporal.Instant.from(value);
+}

src/time/ics.ts

@@ -0,0 +1,114 @@
+import { assertBoundedText } from "@add-ideas/toolbox-helpers";
+import { Temporal } from "temporal-polyfill";
+
+export interface EventInput {
+  startLocal: string;
+  timeZone: string;
+  duration: string;
+  summary: string;
+  description?: string;
+  location?: string;
+  uid?: string;
+}
+
+function escapeText(value: string): string {
+  return value
+    .replaceAll("\\", "\\\\")
+    .replaceAll(";", "\\;")
+    .replaceAll(",", "\\,")
+    .replaceAll("\r\n", "\n")
+    .replaceAll("\r", "\n")
+    .replaceAll("\n", "\\n");
+}
+
+function utcBasic(instant: Temporal.Instant): string {
+  return instant
+    .toString({ smallestUnit: "second" })
+    .replaceAll("-", "")
+    .replaceAll(":", "");
+}
+
+export function foldIcsLine(line: string): string {
+  const encoder = new TextEncoder();
+  const codePoints = [...line];
+  const lines: string[] = [];
+  let current = "";
+  let bytes = 0;
+  for (const value of codePoints) {
+    const length = encoder.encode(value).length;
+    const limit = lines.length === 0 ? 75 : 74;
+    if (bytes + length > limit && current) {
+      lines.push(current);
+      current = value;
+      bytes = length;
+    } else {
+      current += value;
+      bytes += length;
+    }
+  }
+  lines.push(current);
+  return lines.join("\r\n ");
+}
+
+export function createUtcEvent(input: EventInput): {
+  ics: string;
+  startInstant: string;
+  endInstant: string;
+  note: string;
+} {
+  const timeZone = assertBoundedText(input.timeZone, 128, "Time zone").trim();
+  const startLocal = assertBoundedText(
+    input.startLocal,
+    128,
+    "Start local date-time",
+  ).trim();
+  const durationInput = assertBoundedText(
+    input.duration,
+    256,
+    "Duration",
+  ).trim();
+  const start = Temporal.PlainDateTime.from(startLocal).toZonedDateTime(
+    timeZone,
+    { disambiguation: "reject" },
+  );
+  const duration = Temporal.Duration.from(durationInput);
+  const end = start.add(duration);
+  if (end.epochNanoseconds <= start.epochNanoseconds)
+    throw new RangeError(
+      "Event duration must result in an end after the start.",
+    );
+  const summary = assertBoundedText(input.summary, 1_024, "Summary").trim();
+  if (!summary) throw new SyntaxError("Summary is required.");
+  const uid = assertBoundedText(
+    input.uid?.trim() || `${start.epochNanoseconds}@time-tools.local`,
+    512,
+    "UID",
+  );
+  const lines = [
+    "BEGIN:VCALENDAR",
+    "VERSION:2.0",
+    "PRODID:-//add ideas//Time Tools 0.1.0//EN",
+    "CALSCALE:GREGORIAN",
+    "BEGIN:VEVENT",
+    `UID:${escapeText(uid)}`,
+    `DTSTAMP:${utcBasic(Temporal.Now.instant())}`,
+    `DTSTART:${utcBasic(start.toInstant())}`,
+    `DTEND:${utcBasic(end.toInstant())}`,
+    `SUMMARY:${escapeText(summary)}`,
+  ];
+  if (input.description?.trim())
+    lines.push(
+      `DESCRIPTION:${escapeText(assertBoundedText(input.description, 32_768, "Description"))}`,
+    );
+  if (input.location?.trim())
+    lines.push(
+      `LOCATION:${escapeText(assertBoundedText(input.location, 4_096, "Location"))}`,
+    );
+  lines.push("END:VEVENT", "END:VCALENDAR");
+  return {
+    ics: `${lines.map(foldIcsLine).join("\r\n")}\r\n`,
+    startInstant: start.toInstant().toString(),
+    endInstant: end.toInstant().toString(),
+    note: `The event was converted from ${timeZone} to UTC. This avoids an incomplete VTIMEZONE definition but does not preserve a named-zone wall-clock recurrence.`,
+  };
+}

src/time/recurrence.ts

@@ -0,0 +1,101 @@
+import { assertBoundedText } from "@add-ideas/toolbox-helpers";
+import { Cron, type CronMode } from "croner";
+import { RRuleTemporal } from "rrule-temporal";
+import { Temporal } from "temporal-polyfill";
+
+export interface RecurrenceOccurrence {
+  index: number;
+  instant: string;
+  zoned: string;
+  offset: string;
+}
+
+function boundedCount(value: number): number {
+  const count = Math.trunc(value);
+  if (!Number.isSafeInteger(count) || count < 1 || count > 1_000)
+    throw new RangeError("Occurrence count must be between 1 and 1,000.");
+  return count;
+}
+
+export function previewCron(
+  patternInput: string,
+  startInput: string,
+  timeZoneInput: string,
+  countInput: number,
+  mode: CronMode = "auto",
+): RecurrenceOccurrence[] {
+  const pattern = assertBoundedText(
+    patternInput,
+    512,
+    "Cron expression",
+  ).trim();
+  const timeZone = assertBoundedText(timeZoneInput, 128, "Time zone").trim();
+  const count = boundedCount(countInput);
+  const start = Temporal.Instant.from(startInput);
+  const cron = new Cron(pattern, {
+    paused: true,
+    timezone: timeZone,
+    mode,
+    legacyMode: true,
+  });
+  return cron
+    .nextRuns(count, new Date(start.epochMilliseconds))
+    .map((date, index) => {
+      const instant = Temporal.Instant.fromEpochMilliseconds(date.getTime());
+      const zoned = instant.toZonedDateTimeISO(timeZone);
+      return {
+        index: index + 1,
+        instant: instant.toString(),
+        zoned: zoned.toString(),
+        offset: zoned.offset,
+      };
+    });
+}
+
+export function previewRRule(
+  ruleInput: string,
+  startLocalInput: string,
+  timeZoneInput: string,
+  countInput: number,
+): RecurrenceOccurrence[] {
+  let rruleString = assertBoundedText(ruleInput, 4_096, "RRULE")
+    .trim()
+    .replaceAll("\r\n", "\n");
+  if (!rruleString) throw new SyntaxError("RRULE is required.");
+  if (!rruleString.toUpperCase().includes("RRULE:"))
+    rruleString = `RRULE:${rruleString}`;
+  if (
+    /\n(?:DTSTART|RRULE|RDATE|EXDATE)[^\n]*\n(?:DTSTART|RRULE|RDATE|EXDATE)/iu.test(
+      `\n${rruleString}`,
+    )
+  ) {
+    throw new RangeError(
+      "Provide one RRULE. DTSTART is configured separately in this workspace.",
+    );
+  }
+  const count = boundedCount(countInput);
+  const timeZone = assertBoundedText(timeZoneInput, 128, "Time zone").trim();
+  const start = Temporal.PlainDateTime.from(
+    startLocalInput.trim(),
+  ).toZonedDateTime(timeZone, { disambiguation: "compatible" });
+  const rule = new RRuleTemporal({
+    rruleString,
+    dtstart: start,
+    temporal: Temporal,
+    maxIterations: 20_000,
+    maxCandidateEvaluations: 250_000,
+    cache: false,
+    strict: true,
+  });
+  return rule
+    .all((_date, index) => index < count)
+    .map((value, index) => {
+      const zoned = Temporal.ZonedDateTime.from(value.toString());
+      return {
+        index: index + 1,
+        instant: zoned.toInstant().toString(),
+        zoned: zoned.toString(),
+        offset: zoned.offset,
+      };
+    });
+}

src/time/zones.ts

@@ -0,0 +1,138 @@
+import { assertBoundedText } from "@add-ideas/toolbox-helpers";
+import { Temporal } from "temporal-polyfill";
+import { parseInstant, type EpochUnit } from "./epoch";
+
+export interface LocalResolution {
+  status: "exact" | "ambiguous" | "skipped";
+  local: string;
+  timeZone: string;
+  choices: Array<{
+    label: "only" | "earlier" | "later";
+    instant: string;
+    zoned: string;
+    offset: string;
+  }>;
+}
+
+export interface ZoneSnapshot {
+  timeZone: string;
+  local: string;
+  offset: string;
+  epochNanoseconds: string;
+}
+
+export interface ZoneTransition {
+  instant: string;
+  localAfter: string;
+  offsetBefore: string;
+  offsetAfter: string;
+  changeMinutes: number;
+}
+
+function samePlain(
+  left: Temporal.PlainDateTime,
+  right: Temporal.PlainDateTime,
+): boolean {
+  return left.equals(right);
+}
+
+export function resolveLocalDateTime(
+  localInput: string,
+  timeZoneInput: string,
+): LocalResolution {
+  const local = Temporal.PlainDateTime.from(
+    assertBoundedText(localInput, 128, "Local date-time").trim(),
+  );
+  const timeZone = assertBoundedText(timeZoneInput, 128, "Time zone").trim();
+  if (!timeZone) throw new SyntaxError("Time zone is required.");
+  const earlier = local.toZonedDateTime(timeZone, {
+    disambiguation: "earlier",
+  });
+  const later = local.toZonedDateTime(timeZone, { disambiguation: "later" });
+  if (earlier.epochNanoseconds === later.epochNanoseconds) {
+    return {
+      status: "exact",
+      local: local.toString(),
+      timeZone,
+      choices: [
+        {
+          label: "only",
+          instant: earlier.toInstant().toString(),
+          zoned: earlier.toString(),
+          offset: earlier.offset,
+        },
+      ],
+    };
+  }
+  const bothMatch =
+    samePlain(earlier.toPlainDateTime(), local) &&
+    samePlain(later.toPlainDateTime(), local);
+  return {
+    status: bothMatch ? "ambiguous" : "skipped",
+    local: local.toString(),
+    timeZone,
+    choices: [
+      {
+        label: "earlier",
+        instant: earlier.toInstant().toString(),
+        zoned: earlier.toString(),
+        offset: earlier.offset,
+      },
+      {
+        label: "later",
+        instant: later.toInstant().toString(),
+        zoned: later.toString(),
+        offset: later.offset,
+      },
+    ],
+  };
+}
+
+export function compareTimeZones(
+  instantInput: string,
+  zonesInput: string[],
+  numericUnit: EpochUnit = "seconds",
+): ZoneSnapshot[] {
+  if (zonesInput.length === 0 || zonesInput.length > 24)
+    throw new RangeError("Choose between 1 and 24 time zones.");
+  const instant = parseInstant(instantInput, numericUnit);
+  return zonesInput.map((zoneInput) => {
+    const timeZone = assertBoundedText(zoneInput, 128, "Time zone").trim();
+    const zoned = instant.toZonedDateTimeISO(timeZone);
+    return {
+      timeZone: zoned.timeZoneId,
+      local: zoned.toPlainDateTime().toString(),
+      offset: zoned.offset,
+      epochNanoseconds: zoned.epochNanoseconds.toString(),
+    };
+  });
+}
+
+export function nextTransitions(
+  instantInput: string,
+  timeZoneInput: string,
+  countInput = 6,
+): ZoneTransition[] {
+  if (!Number.isSafeInteger(countInput) || countInput < 1 || countInput > 32)
+    throw new RangeError("Transition count must be between 1 and 32.");
+  const count = countInput;
+  const timeZone = assertBoundedText(timeZoneInput, 128, "Time zone").trim();
+  let cursor = parseInstant(instantInput).toZonedDateTimeISO(timeZone);
+  const result: ZoneTransition[] = [];
+  for (let index = 0; index < count; index += 1) {
+    const transition = cursor.getTimeZoneTransition("next");
+    if (!transition) break;
+    const before = transition.subtract({ nanoseconds: 1 });
+    result.push({
+      instant: transition.toInstant().toString(),
+      localAfter: transition.toPlainDateTime().toString(),
+      offsetBefore: before.offset,
+      offsetAfter: transition.offset,
+      changeMinutes:
+        (transition.offsetNanoseconds - before.offsetNanoseconds) /
+        60_000_000_000,
+    });
+    cursor = transition.add({ nanoseconds: 1 });
+  }
+  return result;
+}

src/toolbox/manifest.source.json

@@ -0,0 +1,41 @@
+{
+  "$schema": "https://git.add-ideas.de/lotobo/toolbox-sdk/raw/branch/main/schemas/toolbox-app.v1.schema.json",
+  "schemaVersion": 1,
+  "id": "de.add-ideas.time-tools",
+  "name": "Time Tools",
+  "version": "0.1.0",
+  "description": "Work with dates, time zones and recurrences locally in the browser.",
+  "entry": "./",
+  "icon": "./favicon.svg",
+  "categories": ["time", "developer", "productivity"],
+  "tags": ["timestamp", "timezone", "cron", "rrule", "ics"],
+  "integration": {
+    "contextVersion": 1,
+    "launchModes": ["navigate", "new-tab"],
+    "embedding": "unsupported"
+  },
+  "requirements": {
+    "secureContext": false,
+    "workers": false,
+    "indexedDb": false,
+    "crossOriginIsolated": false,
+    "topLevelContext": false
+  },
+  "privacy": {
+    "processing": "local",
+    "fileUploads": false,
+    "telemetry": false,
+    "label": "Inputs stay in this browser; nothing is uploaded."
+  },
+  "source": {
+    "repository": "https://git.add-ideas.de/lotobo/time-tools",
+    "license": "GPL-3.0-or-later"
+  },
+  "actions": [
+    {
+      "id": "source",
+      "label": "Source",
+      "url": "https://git.add-ideas.de/lotobo/time-tools"
+    }
+  ]
+}

src/toolbox/manifest.ts

@@ -0,0 +1,4 @@
+import { defineToolboxApp, parseToolboxApp } from "@add-ideas/toolbox-contract";
+import source from "./manifest.source.json";
+
+export const manifest = defineToolboxApp(parseToolboxApp(source));

src/version.ts

@@ -0,0 +1 @@
+export const APP_VERSION = "0.1.0";

src/vite-env.d.ts

@@ -0,0 +1 @@
+/// <reference types="vite/client" />

tests/browser/app.spec.ts

@@ -0,0 +1,61 @@
+import { expect, test, type Page } from "@playwright/test";
+
+const ORIGIN = "http://127.0.0.1:4173";
+async function localOnly(page: Page) {
+  const external: string[] = [];
+  await page.route("**/*", async (route) => {
+    const url = new URL(route.request().url());
+    if (url.origin !== ORIGIN) {
+      external.push(url.href);
+      await route.abort();
+    } else await route.continue();
+  });
+  return external;
+}
+
+test("runs from a nested path without external requests", async ({ page }) => {
+  const errors: string[] = [];
+  page.on("pageerror", (error) => errors.push(error.message));
+  page.on("console", (message) => {
+    if (message.type() === "error") errors.push(message.text());
+  });
+  const external = await localOnly(page);
+  await page.goto("/deep/nested/time/");
+  await expect(page.getByRole("heading", { name: "Time Tools" })).toBeVisible();
+  expect(external).toEqual([]);
+  expect(errors).toEqual([]);
+});
+
+test("converts a negative nanosecond epoch exactly", async ({ page }) => {
+  const external = await localOnly(page);
+  await page.goto("/deep/nested/time/");
+  await page.getByLabel("Epoch value").fill("-0.000000001");
+  await page.getByRole("button", { name: "Convert exactly" }).click();
+  await expect(
+    page.getByRole("heading", { name: "Exact timestamp conversion" }),
+  ).toBeVisible();
+  await expect(page.locator(".result > pre")).toContainText(
+    "1969-12-31T23:59:59.999999999Z",
+  );
+  await expect(page.locator(".result > pre")).toContainText(
+    '"epochNanoseconds": "-1"',
+  );
+  expect(external).toEqual([]);
+});
+
+test("serves the release identity and hardened headers", async ({
+  request,
+}) => {
+  const index = await request.get("/deep/nested/time/");
+  expect(index.ok()).toBe(true);
+  expect(index.headers()["content-security-policy"]).toContain(
+    "default-src 'self'",
+  );
+  expect(await index.text()).not.toMatch(/\b(?:src|href)=["']\//u);
+  const manifest = await request.get("/deep/nested/time/toolbox-app.json");
+  await expect(manifest.json()).resolves.toMatchObject({
+    id: "de.add-ideas.time-tools",
+    version: "0.1.0",
+    entry: "./",
+  });
+});

tests/components/app.test.tsx

@@ -0,0 +1,17 @@
+import { render, screen } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+import { App } from "../../src/App";
+
+describe("Time Tools", () => {
+  it("renders the local workbench and standard shell", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => new Response("Not found", { status: 404 })),
+    );
+    render(<App />);
+    expect(
+      await screen.findByRole("heading", { name: "Time Tools" }),
+    ).toBeVisible();
+    expect(await screen.findByText("No network requests")).toBeVisible();
+  });
+});

tests/time/core.test.ts

@@ -0,0 +1,131 @@
+import { describe, expect, it } from "vitest";
+import { addBusinessDays, compareArithmetic } from "../../src/time/arithmetic";
+import { inspectEpoch } from "../../src/time/epoch";
+import { createUtcEvent, foldIcsLine } from "../../src/time/ics";
+import { previewCron, previewRRule } from "../../src/time/recurrence";
+import {
+  compareTimeZones,
+  nextTransitions,
+  resolveLocalDateTime,
+} from "../../src/time/zones";
+
+describe("exact epoch conversion", () => {
+  it("preserves negative nanoseconds", () => {
+    const result = inspectEpoch("-0.000000001", "seconds");
+    expect(result.epochNanoseconds).toBe(-1n);
+    expect(result.iso).toBe("1969-12-31T23:59:59.999999999Z");
+  });
+
+  it("rejects sub-nanosecond precision", () => {
+    expect(() => inspectEpoch("0.0000000001", "seconds")).toThrow(
+      /smaller than one nanosecond/u,
+    );
+  });
+});
+
+describe("time-zone handling", () => {
+  it("classifies a spring gap and autumn overlap", () => {
+    expect(
+      resolveLocalDateTime("2026-03-29T02:30", "Europe/Berlin").status,
+    ).toBe("skipped");
+    const overlap = resolveLocalDateTime("2026-10-25T02:30", "Europe/Berlin");
+    expect(overlap.status).toBe("ambiguous");
+    expect(overlap.choices.map((choice) => choice.offset)).toEqual([
+      "+02:00",
+      "+01:00",
+    ]);
+  });
+
+  it("compares the same instant without changing its epoch", () => {
+    const values = compareTimeZones("2026-01-01T00:00:00Z", [
+      "UTC",
+      "Asia/Tokyo",
+    ]);
+    expect(values[0]?.epochNanoseconds).toBe(values[1]?.epochNanoseconds);
+    expect(values[1]?.local).toBe("2026-01-01T09:00:00");
+  });
+
+  it("rejects a non-integer transition count", () => {
+    expect(() =>
+      nextTransitions("2026-01-01T00:00:00Z", "Europe/Berlin", 1.5),
+    ).toThrow(/between 1 and 32/u);
+  });
+});
+
+describe("calendar arithmetic", () => {
+  it("shows wall-clock and elapsed days diverging across DST", () => {
+    const result = compareArithmetic(
+      "2026-03-28T12:00:00Z",
+      "Europe/Berlin",
+      "P1D",
+    );
+    expect(result.differenceSeconds).toBe("-3600");
+  });
+
+  it("adds business days with explicit holidays", () => {
+    expect(addBusinessDays("2026-12-24", 2, ["2026-12-25"]).result).toBe(
+      "2026-12-29",
+    );
+  });
+
+  it("bounds explicit holiday allocation", () => {
+    expect(() =>
+      addBusinessDays("2026-01-01", 1, Array(10_001).fill("2026-01-02")),
+    ).toThrow(/10,000/u);
+  });
+
+  it("does not silently truncate a fractional business-day count", () => {
+    expect(() => addBusinessDays("2026-01-01", 1.5)).toThrow(/±100,000/u);
+  });
+});
+
+describe("bounded recurrence previews", () => {
+  it("previews five-field cron in its named zone", () => {
+    const result = previewCron(
+      "0 9 * * MON-FRI",
+      "2026-08-31T08:00:00Z",
+      "Europe/Berlin",
+      2,
+      "5-part",
+    );
+    expect(result).toHaveLength(2);
+    expect(result[0]?.zoned).toContain("T09:00:00");
+  });
+
+  it("previews a strict RRULE", () => {
+    const result = previewRRule(
+      "FREQ=DAILY;COUNT=3",
+      "2026-09-01T09:00",
+      "Europe/Berlin",
+      10,
+    );
+    expect(result).toHaveLength(3);
+    expect(result[0]?.zoned).toContain("2026-09-01T09:00:00");
+  });
+});
+
+describe("iCalendar output", () => {
+  it("creates CRLF UTC event lines", () => {
+    const result = createUtcEvent({
+      startLocal: "2026-09-01T10:00",
+      timeZone: "Europe/Berlin",
+      duration: "PT1H",
+      summary: "Review, plan",
+    });
+    expect(result.ics).toContain("DTSTART:20260901T080000Z\r\n");
+    expect(result.ics).toContain("SUMMARY:Review\\, plan");
+    expect(result.ics.endsWith("\r\n")).toBe(true);
+  });
+
+  it("folds Unicode lines by UTF-8 octets", () => {
+    const folded = foldIcsLine(`DESCRIPTION:${"ä".repeat(50)}`);
+    expect(folded).toContain("\r\n ");
+    expect(
+      Math.max(
+        ...folded
+          .split("\r\n")
+          .map((line) => new TextEncoder().encode(line).length),
+      ),
+    ).toBeLessThanOrEqual(75);
+  });
+});

tsconfig.app.json

@@ -0,0 +1,27 @@
+{
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.app.tsbuildinfo",
+    "target": "ES2023",
+    "lib": ["ES2023", "DOM", "DOM.Iterable"],
+    "module": "ESNext",
+    "types": ["vite/client"],
+    "allowArbitraryExtensions": true,
+    "skipLibCheck": true,
+    "moduleResolution": "Bundler",
+    "allowImportingTsExtensions": true,
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+    "jsx": "react-jsx",
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "erasableSyntaxOnly": true,
+    "noFallthroughCasesInSwitch": true,
+    "strict": true,
+    "noUncheckedIndexedAccess": true,
+    "useDefineForClassFields": true,
+    "resolveJsonModule": true,
+    "isolatedModules": true
+  },
+  "include": ["src", "tests"]
+}

tsconfig.json

@@ -0,0 +1,11 @@
+{
+  "files": [],
+  "references": [
+    {
+      "path": "./tsconfig.app.json"
+    },
+    {
+      "path": "./tsconfig.node.json"
+    }
+  ]
+}

tsconfig.node.json

@@ -0,0 +1,24 @@
+{
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.node.tsbuildinfo",
+    "target": "ES2023",
+    "lib": ["ES2023"],
+    "module": "ESNext",
+    "types": ["node"],
+    "skipLibCheck": true,
+    "moduleResolution": "Bundler",
+    "allowImportingTsExtensions": true,
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true
+  },
+  "include": [
+    "vite.config.ts",
+    "playwright.config.ts",
+    "eslint.config.mjs",
+    "scripts/**/*.mjs"
+  ]
+}

vite.config.ts

@@ -0,0 +1,16 @@
+/// <reference types="vitest/config" />
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+
+export default defineConfig({
+  base: "./",
+  plugins: [react()],
+  test: {
+    environment: "jsdom",
+    setupFiles: "./src/test/setup.ts",
+    css: true,
+    maxWorkers: 2,
+    restoreMocks: true,
+    exclude: ["tests/browser/**", "node_modules/**", "dist/**"],
+  },
+});