refactor(webui): use Core date-time formatting

webui/src/features/scheduling/SchedulingPage.tsx

@@ -18,6 +18,7 @@ import {
   DataGridEmptyAction,
   DataGridRowActions,
   DateTimeField,
+  formatDateTime,
   ModuleSubnav,
   SelectionList,
   SelectionListItem,
@@ -31,6 +32,7 @@ import {
   type AuthInfo,
   type CalendarPickerUiCapability,
   type DataGridColumn,
+  type FormatDateTimeOptions,
   type ModuleSubnavGroup
 } from "@govoplan/core-webui";
 import {
@@ -1064,8 +1066,8 @@ function CandidateSlotsGrid({
         </span>
       )
     },
-    { id: "start", header: I18N.start, minWidth: 160, resizable: true, value: (slot) => slot.start_at, render: (slot) => formatDateTime(slot.start_at) },
-    { id: "end", header: I18N.end, minWidth: 160, resizable: true, value: (slot) => slot.end_at, render: (slot) => formatDateTime(slot.end_at) },
+    { id: "start", header: I18N.start, minWidth: 160, resizable: true, value: (slot) => slot.start_at, render: (slot) => formatDateTime(slot.start_at, SHORT_DATE_TIME_OPTIONS) },
+    { id: "end", header: I18N.end, minWidth: 160, resizable: true, value: (slot) => slot.end_at, render: (slot) => formatDateTime(slot.end_at, SHORT_DATE_TIME_OPTIONS) },
     {
       id: "available",
       header: I18N.available,
@@ -1235,17 +1237,24 @@ function isoFromLocal(value: string): string {
   return new Date(value).toISOString();
 }
 
-function formatDateTime(value: string): string {
-  return new Intl.DateTimeFormat(undefined, { dateStyle: "short", timeStyle: "short" }).format(new Date(value));
-}
+const SHORT_DATE_TIME_OPTIONS: FormatDateTimeOptions = {
+  year: undefined,
+  month: undefined,
+  day: undefined,
+  hour: undefined,
+  minute: undefined,
+  timeZoneName: undefined,
+  dateStyle: "short",
+  timeStyle: "short"
+};
 
 function formatRange(start: string, end: string): string {
-  return `${formatDateTime(start)} – ${formatDateTime(end)}`;
+  return `${formatDateTime(start, SHORT_DATE_TIME_OPTIONS)} – ${formatDateTime(end, SHORT_DATE_TIME_OPTIONS)}`;
 }
 
 function formatRelevantDate(request: SchedulingRequest): string {
   const timestamp = schedulingRelevantTimestamp(request);
-  return Number.isFinite(timestamp) ? formatDateTime(new Date(timestamp).toISOString()) : "";
+  return Number.isFinite(timestamp) ? formatDateTime(new Date(timestamp).toISOString(), SHORT_DATE_TIME_OPTIONS) : "";
 }
 
 function errorMessage(error: unknown, fallback: string): string {