fix: use UTC timestamps for timed event rendering

lib/__tests__/calendar-utils.test.ts

@@ -7,6 +7,7 @@ import {
   getEventDayBounds,
   getEventDisplayEndDate,
   getEventEndDate,
+  getEventStartDate,
   getTimedEventBoundsForDay,
   isTimedEventFullDayOnDate,
   layoutOverlappingEvents,
@@ -96,7 +97,21 @@ describe('calendar-utils all-day handling', () => {
       utcEnd: '2026-03-14T11:00:00Z',
     });
 
-    expectLocalDateParts(getEventDisplayEndDate(event), 2026, 3, 14, 11);
+    expect(getEventDisplayEndDate(event).toISOString()).toBe('2026-03-14T11:00:00.000Z');
+  });
+
+  it('prefers utc timestamps for timed events with an event timezone', () => {
+    const event = makeEvent({
+      start: '2026-03-15T09:00:00',
+      duration: 'PT1H',
+      timeZone: 'America/New_York',
+      showWithoutTime: false,
+      utcStart: '2026-03-15T13:00:00Z',
+      utcEnd: '2026-03-15T14:00:00Z',
+    });
+
+    expect(getEventStartDate(event).toISOString()).toBe('2026-03-15T13:00:00.000Z');
+    expect(getEventEndDate(event).toISOString()).toBe('2026-03-15T14:00:00.000Z');
   });
 
   it('clips timed multi-day events to the visible day bounds', () => {
@@ -109,7 +124,7 @@ describe('calendar-utils all-day handling', () => {
     });
 
     expect(getTimedEventBoundsForDay(event, new Date('2026-03-14T00:00:00Z'))).toMatchObject({
-      startMinutes: 1320,
+      startMinutes: 1380,
       endMinutes: 1440,
       continuesBefore: false,
       continuesAfter: true,
@@ -117,7 +132,7 @@ describe('calendar-utils all-day handling', () => {
 
     expect(getTimedEventBoundsForDay(event, new Date('2026-03-15T00:00:00Z'))).toMatchObject({
       startMinutes: 0,
-      endMinutes: 120,
+      endMinutes: 180,
       continuesBefore: true,
       continuesAfter: false,
     });
@@ -137,7 +152,7 @@ describe('calendar-utils all-day handling', () => {
     expect(layout).toHaveLength(1);
     expect(layout[0]).toMatchObject({
       startMinutes: 0,
-      endMinutes: 120,
+      endMinutes: 180,
       column: 0,
       totalColumns: 1,
       continuesBefore: true,

lib/calendar-utils.ts

@@ -21,6 +21,13 @@ export interface TimedEventLayout {
   continuesAfter: boolean;
 }
 
+export function getEventStartDate(
+  event: Pick<CalendarEvent, 'start' | 'utcStart' | 'showWithoutTime'>,
+): Date {
+  const source = !event.showWithoutTime && event.utcStart ? event.utcStart : event.start;
+  return parseISO(source);
+}
+
 export function packWeekSegments(rawSegments: CalendarWeekSegment[]): CalendarWeekSegment[] {
   rawSegments.sort((left, right) => {
     if (left.startIndex !== right.startIndex) return left.startIndex - right.startIndex;
@@ -28,7 +35,7 @@ export function packWeekSegments(rawSegments: CalendarWeekSegment[]): CalendarWe
     if (left.event.showWithoutTime !== right.event.showWithoutTime) {
       return left.event.showWithoutTime ? -1 : 1;
     }
-    const timeDiff = parseISO(left.event.start).getTime() - parseISO(right.event.start).getTime();
+    const timeDiff = getEventStartDate(left.event).getTime() - getEventStartDate(right.event).getTime();
     if (timeDiff !== 0) return timeDiff;
     return (left.event.title || "").localeCompare(right.event.title || "");
   });
@@ -48,14 +55,19 @@ export function packWeekSegments(rawSegments: CalendarWeekSegment[]): CalendarWe
 }
 
 export function getEventEndDate(event: CalendarEvent): Date {
-  const start = parseISO(event.start);
+  if (!event.showWithoutTime && event.utcEnd) {
+    return parseISO(event.utcEnd);
+  }
+
+  const start = getEventStartDate(event);
   if (!event.duration) return start;
   return new Date(start.getTime() + parseDuration(event.duration) * 60000);
 }
 
 export function getEventDisplayEndDate(event: CalendarEvent): Date {
   const end = getEventEndDate(event);
-  if (!event.showWithoutTime || end.getTime() <= parseISO(event.start).getTime()) {
+  const start = getEventStartDate(event);
+  if (!event.showWithoutTime || end.getTime() <= start.getTime()) {
     return end;
   }
   return subMilliseconds(end, 1);
@@ -63,7 +75,7 @@ export function getEventDisplayEndDate(event: CalendarEvent): Date {
 
 export function getEventDayBounds(event: CalendarEvent): { startDay: Date; endDay: Date } {
   return {
-    startDay: startOfDay(parseISO(event.start)),
+    startDay: startOfDay(getEventStartDate(event)),
     endDay: startOfDay(getEventDisplayEndDate(event)),
   };
 }
@@ -74,7 +86,7 @@ export function getTimedEventBoundsForDay(
 ): { startMinutes: number; endMinutes: number; continuesBefore: boolean; continuesAfter: boolean } | null {
   if (event.showWithoutTime) return null;
 
-  const eventStart = parseISO(event.start);
+  const eventStart = getEventStartDate(event);
   const eventEnd = getEventEndDate(event);
   const dayStart = startOfDay(day);
   const nextDayStart = addDays(dayStart, 1);