fix: end a zero-length event at its start instead of at midnight

src/lib/__tests__/calendar-utils.test.ts

@@ -332,6 +332,7 @@ describe('getTimedEventBoundsForDay on DST change days', () => {
         timeZone: 'Europe/Berlin',
         utcStart: '2026-10-25T11:51:00Z',
         utcEnd: '2026-10-25T12:16:00Z',
+        duration: 'PT25M',
       });
       expect(getTimedEventBoundsForDay(fallBack, new Date(2026, 9, 25))).toMatchObject({
         startMinutes: 12 * 60 + 51,
@@ -344,6 +345,7 @@ describe('getTimedEventBoundsForDay on DST change days', () => {
         timeZone: 'Europe/Berlin',
         utcStart: '2026-03-29T07:00:00Z',
         utcEnd: '2026-03-29T08:00:00Z',
+        duration: 'PT1H',
       });
       expect(getTimedEventBoundsForDay(springForward, new Date(2026, 2, 29))).toMatchObject({
         startMinutes: 9 * 60,
@@ -354,3 +356,13 @@ describe('getTimedEventBoundsForDay on DST change days', () => {
     }
   });
 });
+
+describe('getEventEndDate', () => {
+  // #837: Stalwart stores a DTEND equal to DTSTART as no duration and ends
+  // the event at 23:59:59 of its day.
+  it('ends a timed event without a duration at its start, whatever utcEnd says', () => {
+    const zero = ev({ start: '2026-03-02T09:00:00', utcStart: '2026-03-02T09:00:00Z', utcEnd: '2026-03-02T23:59:59Z' });
+    delete (zero as Partial<CalendarEvent>).duration;
+    expect(getEventEndDate(zero).getTime()).toBe(getEventStartDate(zero).getTime());
+  });
+});

src/lib/calendar-utils.ts

@@ -113,7 +113,11 @@ export function addDaysToLocalDateTime(value: string, days: number): string {
 
 /** End of an event as a display date - see getEventStartDate. */
 export function getEventEndDate(event: CalendarEvent): Date {
-  if (!event.showWithoutTime && event.utcEnd) {
+  // A timed event without a duration lasts zero seconds (RFC 8984's default
+  // PT0S). Stalwart stores a DTEND equal to DTSTART as no duration and gives
+  // the event a utcEnd of 23:59:59 on its start day, so it filled the rest
+  // of the day in the grid (#837).
+  if (!event.showWithoutTime && event.utcEnd && event.duration) {
     const utc = parseISO(event.utcEnd);
     if (!isNaN(utc.getTime())) return toDisplayDate(utc);
   }