fix: show all-day events on their own day in the Calendar Agenda for viewers west of the server's time zone

app/api/calendar-agenda/route.ts

@@ -52,6 +52,7 @@ interface AgendaEvent {
   accountId: string;
   uid: string | null;
   title: string;
+  /** An instant (`...Z`); for an all-day event the date-time without an offset. */
   start: string;
   end: string;
   allDay: boolean;
@@ -97,6 +98,20 @@ function eventEnd(event: Partial<CalendarEvent>): Date {
   return new Date(start.getTime() + parseDurationMinutes(event.duration) * 60000);
 }
 
+/**
+ * An all-day event's start and end as the dates it is stored with
+ * (`2026-10-27T00:00:00`, no offset). As an instant it would be midnight in
+ * this server's time zone, which anyone west of the server reads as the day
+ * before; `new Date()` reads a date-time without an offset as the viewer's
+ * own local time.
+ */
+function allDayRange(event: Partial<CalendarEvent>): { start: string; end: string } {
+  const start = (event.start as string).slice(0, 19);
+  const startMs = Date.parse(`${start}Z`);
+  const end = new Date(startMs + parseDurationMinutes(event.duration) * 60000).toISOString().slice(0, 19);
+  return { start, end };
+}
+
 function firstLocationName(event: Partial<CalendarEvent>): string | null {
   const locations = event.locations;
   if (!locations || typeof locations !== 'object') return null;
@@ -318,8 +333,9 @@ export async function POST(request: NextRequest) {
           accountId: e.accountId,
           uid: e.uid ?? null,
           title: (e.title ?? '').trim() || '(no title)',
-          start: eventStart(e).toISOString(),
-          end: eventEnd(e).toISOString(),
+          ...(e.showWithoutTime
+            ? allDayRange(e)
+            : { start: eventStart(e).toISOString(), end: eventEnd(e).toISOString() }),
           allDay: !!e.showWithoutTime,
           status: e.status ?? null,
           color: e.color || cal?.color || null,

lib/__tests__/calendar-agenda-route.test.ts

@@ -240,6 +240,28 @@ describe.each<StalwartVersion>(['0.16', '1.0'])('calendar-agenda route on Stalwa
     expect(events[0]).toMatchObject({ start: '2026-09-26T08:00:00.000Z', end: '2026-09-26T09:00:00.000Z' });
   });
 
+  it(`hands out an all-day event as its own date, not midnight in the server's zone, on Stalwart ${version}`, async () => {
+    // As the server answered live: 0.16 drops showWithoutTime and names
+    // Etc/UTC, 1.0 keeps showWithoutTime and names no zone.
+    const holiday = {
+      id: 'maaaaac', baseEventId: 'c', '@type': 'Event', uid: 'c@example.com', title: 'Holiday',
+      start: '2026-09-27T00:00:00', duration: 'P1D',
+      utcStart: '2026-09-27T00:00:00Z', utcEnd: '2026-09-28T00:00:00Z', calendarIds: { cal: true },
+      ...(version === '1.0' ? { showWithoutTime: true } : { timeZone: 'Etc/UTC' }),
+    };
+    answer = ({ method, args }) => method === 'CalendarEvent/get' && (args.ids as string[]).includes('maaaaac')
+      ? ['CalendarEvent/get', { list: [OCCURRENCES.maaaaab, holiday] }]
+      : undefined;
+    const { events } = await (await POST(makeReq())).json() as { events: Array<Record<string, unknown>> };
+
+    // The plugin files events under new Date(start) in the viewer's zone. An
+    // instant for midnight in the server's zone put the holiday on 26
+    // September for anyone west of the server; a date-time without an offset
+    // is read as the viewer's own midnight.
+    expect(events[1]).toMatchObject({ allDay: true, start: '2026-09-27T00:00:00', end: '2026-09-28T00:00:00' });
+    expect(new Date(events[1].start as string).getDate()).toBe(27);
+  });
+
   it(`answers 502 when a get fails on Stalwart ${version}`, async () => {
     answer = ({ method }) => method === 'CalendarEvent/get' ? ['error', { type: 'serverFail', description: 'Store unavailable' }] : undefined;