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

src/api/__tests__/calendar.test.ts

@@ -395,6 +395,22 @@ describe('calendar operations', () => {
   });
 
   describe('getEvents', () => {
+    // #837: Stalwart stores a DTEND equal to DTSTART as no duration and ends
+    // the event at 23:59:59 of its start day.
+    it('ends a timed event without a duration at its start, and leaves tasks alone', async () => {
+      mockRequest.mockResolvedValue(getAnswer([
+        { id: 'z', '@type': 'Event', start: '2026-03-02T09:00:00', utcStart: '2026-03-02T08:00:00Z', utcEnd: '2026-03-02T22:59:59Z' },
+        { id: 't', '@type': 'Task', start: '2026-03-02T09:00:00', utcStart: '2026-03-02T08:00:00Z', due: '2026-03-03T09:00:00' },
+        { id: 'a', '@type': 'Event', start: '2026-03-02T00:00:00', showWithoutTime: true },
+      ]));
+
+      const [zero, task, allDay] = await getEvents(['z', 't', 'a']);
+
+      expect(zero).toMatchObject({ duration: 'PT0S', utcEnd: '2026-03-02T08:00:00Z' });
+      expect(task).not.toHaveProperty('duration');
+      expect(allDay).not.toHaveProperty('duration');
+    });
+
     it('should fetch events by id with singular JSCalendar 2.0 recurrence properties', async () => {
       // Stalwart (calcard) names the property `recurrenceRule` (singular,
       // single object) — requesting the RFC 8984 plural form returns no

src/api/__tests__/live-stalwart-calendar.test.ts

@@ -698,6 +698,33 @@ live('live Stalwart calendar', () => {
     }, 60_000);
   });
 
+  describe('a zero-length event (#837)', () => {
+    it('ends at its start, stored without a duration or imported from an .ics', async () => {
+      await connectSingleton();
+      // As another client stores it: no duration at all.
+      const [[, made]] = await raw(client, [['CalendarEvent/set', {
+        accountId: client.accountId,
+        create: { z: { '@type': 'Event', title: `${RUN} zero`, start: '2027-12-01T09:00:00', timeZone: 'Europe/Berlin', calendarIds: { [calendarId]: true } } },
+      }, '0']]);
+      const id = made.created?.z?.id;
+      expect(id, JSON.stringify(made.notCreated)).toBeTruthy();
+      const [read] = await getEvents([id]);
+      expect(read.utcStart).toBe('2027-12-01T08:00:00Z');
+      expect(read.utcEnd).toBe(read.utcStart);
+
+      // An .ics whose DTEND equals its DTSTART, imported.
+      const uid = `${RUN}-zero-import`;
+      await useCalendarStore.getState().fetchCalendars();
+      const { imported } = await useCalendarStore.getState().importEvents([
+        { uid, title: `${RUN} zero imported`, start: '2027-12-02T09:00:00', timeZone: 'Europe/Berlin' },
+      ], calendarId);
+      expect(imported).toBe(1);
+      const [stored] = await seriesByUid(client, uid);
+      const [[, got]] = await raw(client, [['CalendarEvent/get', { accountId: client.accountId, ids: [stored.id], properties: ['duration', 'utcStart', 'utcEnd'] }, '0']]);
+      expect(got.list[0].utcEnd).toBe(got.list[0].utcStart);
+    }, 60_000);
+  });
+
   describe('a subscribed feed', () => {
     const FEED_URL = 'https://feeds.invalid/live.ics';
     let feed = '';

src/api/calendar.ts

@@ -3,7 +3,7 @@ import { CAPABILITIES } from './types';
 import type { Calendar, CalendarEvent, CalendarRights } from './types';
 import { JMAPSetError, assertSetResult, batched, jmapErrorType, requireMethodResult } from './jmap-result';
 import { formatWallClock, getEffectiveTimeZone, getTimeZoneOffsetMs } from '../lib/calendar-timezone';
-import { SCAN_PROPERTIES, type ScannedCalendarObject } from '../lib/calendar-component-detection';
+import { SCAN_PROPERTIES, isTaskLikeObject, type ScannedCalendarObject } from '../lib/calendar-component-detection';
 import {
   RECURRENCE_BASE_PROPERTIES,
   SYNTHETIC_ID_PROBE,
@@ -302,6 +302,18 @@ export function normalizeTaskProgress<T extends Partial<CalendarEvent>>(event: T
     : { ...event, progress: normalized };
 }
 
+/**
+ * A timed event without a duration lasts zero seconds (RFC 8984's default
+ * PT0S). Stalwart drops a DTEND equal to DTSTART and then ends such an event
+ * at 23:59:59 of its start day, so its utcEnd is wrong (#837). Tasks carry
+ * no duration and are left alone. Mirrors the webmail's withZeroDurationEnd.
+ */
+export function withZeroDurationEnd<T extends Partial<CalendarEvent>>(event: T): T {
+  if (event.showWithoutTime || event.duration || !event.start || !event.utcStart) return event;
+  if (isTaskLikeObject(event as ScannedCalendarObject)) return event;
+  return { ...event, duration: 'PT0S', utcEnd: event.utcStart };
+}
+
 /**
  * Convert the client's plural recurrence arrays to the singular JSCalendar 2.0
  * properties Stalwart expects on write, dropping null rule fields. An empty
@@ -970,7 +982,8 @@ export async function getEvents(
     options?.expanded ? EXPANDED_EVENT_PROPERTIES : CALENDAR_EVENT_PROPERTIES,
     { withOverrides: !options?.expanded, timeZone },
   ));
-  return (list as unknown as CalendarEvent[]).map((e) => normalizeTaskProgress(normalizeRecurrenceProperties(e)));
+  return (list as unknown as CalendarEvent[]).map((e) =>
+    withZeroDurationEnd(normalizeTaskProgress(normalizeRecurrenceProperties(e))));
 }
 
 export type EventIdentityFields = Pick<CalendarEvent, 'id' | 'uid' | 'calendarIds'>;

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);
   }

src/stores/__tests__/calendar-store.test.ts

@@ -47,6 +47,7 @@ import {
   ImportRefusedError,
   CalendarAccountChangedError,
   loadEventsInRange,
+  prepareImportedEvent,
 } from '../calendar-store';
 
 const mockGetCalendars = calendarApi.getCalendars as ReturnType<typeof vi.fn>;
@@ -1445,3 +1446,13 @@ describe('editing one occurrence of a series', () => {
     expect(mockUpdateEvent).toHaveBeenCalledTimes(1);
   });
 });
+
+describe('prepareImportedEvent', () => {
+  // #837: CalendarEvent/parse drops a DTEND equal to DTSTART, and stored
+  // without a duration Stalwart ends the event at 23:59:59.
+  it('imports a timed event without a duration as lasting zero seconds', () => {
+    expect(prepareImportedEvent({ uid: 'z', start: '2026-03-02T09:00:00', timeZone: 'Europe/Berlin' }).duration).toBe('PT0S');
+    expect(prepareImportedEvent({ uid: 'h', start: '2026-03-02T09:00:00', duration: 'PT1H' }).duration).toBe('PT1H');
+    expect(prepareImportedEvent({ uid: 't', '@type': 'Task', start: '2026-03-02T09:00:00' })).not.toHaveProperty('duration');
+  });
+});

src/stores/calendar-store.ts

@@ -141,7 +141,12 @@ export function prepareImportedEvent(event: Partial<CalendarEvent>): Partial<Cal
     title: src.title,
     description: src.description,
     start: src.start,
-    duration: src.showWithoutTime ? normalizeAllDayDuration(src.duration) : src.duration,
+    // A timed event without a duration lasts zero seconds. CalendarEvent/parse
+    // drops a DTEND equal to DTSTART, and stored without a duration Stalwart
+    // ends the event at 23:59:59 (#837). Tasks have no duration.
+    duration: src.showWithoutTime
+      ? normalizeAllDayDuration(src.duration)
+      : src.duration || (src['@type'] !== 'Task' && src.start ? 'PT0S' : undefined),
     timeZone: src.showWithoutTime ? null : src.timeZone,
     showWithoutTime: src.showWithoutTime,
     status: src.status,