diff --git a/webui/src/features/addressbook/AddressBookPage.tsx b/webui/src/features/addressbook/AddressBookPage.tsx
index 05ddc4b..d02a294 100644
--- a/webui/src/features/addressbook/AddressBookPage.tsx
+++ b/webui/src/features/addressbook/AddressBookPage.tsx
@@ -8,6 +8,7 @@ import {
   DismissibleAlert,
   ExplorerTree,
   fetchAuthGroups,
+  formatDateTime,
   FormField,
   LoadingFrame,
   PasswordField,
@@ -19,7 +20,8 @@ import {
   hasScope,
   type ApiSettings,
   type AuthInfo,
-  type AuthUpdate
+  type AuthUpdate,
+  type FormatDateTimeOptions
 } from "@govoplan/core-webui";
 import {
   createAddressBook,
@@ -291,14 +293,17 @@ function syncSourceLabel(source: AddressSyncSource): string {
   return source.display_name || source.external_address_book_ref || source.connector_type;
 }
 
-function formatDateTime(value?: string | null): string {
-  if (!value) return "Never";
-  try {
-    return new Intl.DateTimeFormat(undefined, { dateStyle: "short", timeStyle: "short" }).format(new Date(value));
-  } catch {
-    return value;
-  }
-}
+const ADDRESS_DATE_TIME_OPTIONS: FormatDateTimeOptions = {
+  fallback: "Never",
+  year: undefined,
+  month: undefined,
+  day: undefined,
+  hour: undefined,
+  minute: undefined,
+  timeZoneName: undefined,
+  dateStyle: "short",
+  timeStyle: "short"
+};
 
 function planSummary(plan: AddressSyncPlan | null): string {
   if (!plan) return "No preview loaded.";
@@ -1740,7 +1745,7 @@ export default function AddressBookPage({ settings, auth, onAuthChange }: Props)
             {selectedBookSyncSources.map((source) => (
               <p key={source.id}>
                 {syncSourceLabel(source)} · {source.connector_type} · {source.status}
-                {source.last_success_at ? ` · last success ${formatDateTime(source.last_success_at)}` : ""}
+                {source.last_success_at ? ` · last success ${formatDateTime(source.last_success_at, ADDRESS_DATE_TIME_OPTIONS)}` : ""}
                 {source.last_error ? ` · ${source.last_error}` : ""}
               </p>
             ))}
@@ -2314,7 +2319,7 @@ export default function AddressBookPage({ settings, auth, onAuthChange }: Props)
                 <StatusBadge status={syncInspector.source.status} />
                 {syncInspector.source.read_only && <StatusBadge status="read-only" />}
               </div>
-              <p className="muted">Last attempt: {formatDateTime(syncInspector.source.last_attempted_at)} · Last success: {formatDateTime(syncInspector.source.last_success_at)}</p>
+              <p className="muted">Last attempt: {formatDateTime(syncInspector.source.last_attempted_at, ADDRESS_DATE_TIME_OPTIONS)} · Last success: {formatDateTime(syncInspector.source.last_success_at, ADDRESS_DATE_TIME_OPTIONS)}</p>
               {syncInspector.source.last_error && <DismissibleAlert tone="danger" resetKey={syncInspector.source.last_error}>{syncInspector.source.last_error}</DismissibleAlert>}
               <div className="button-row compact-actions">
                 <Button
@@ -2383,7 +2388,7 @@ export default function AddressBookPage({ settings, auth, onAuthChange }: Props)
                 <div className="address-sync-record-list">
                   {syncTombstones.slice(0, 20).map((tombstone) => (
                     <div className="address-sync-record-row" key={tombstone.id}>
-                      <span><strong>{tombstone.remote_uid || tombstone.resource_href || tombstone.contact_id}</strong><small>{formatDateTime(tombstone.synced_at || tombstone.remote_deleted_at || tombstone.local_deleted_at)}</small></span>
+                      <span><strong>{tombstone.remote_uid || tombstone.resource_href || tombstone.contact_id}</strong><small>{formatDateTime(tombstone.synced_at || tombstone.remote_deleted_at || tombstone.local_deleted_at, ADDRESS_DATE_TIME_OPTIONS)}</small></span>
                     </div>
                   ))}
                 </div>