fix: use UTC timestamps for timed event rendering

app/[locale]/calendar/page.tsx

@@ -39,6 +39,7 @@ import { InlineAppView } from "@/components/layout/inline-app-view";
 import { useSidebarApps } from "@/hooks/use-sidebar-apps";
 import { ResizeHandle } from "@/components/layout/resize-handle";
 import { sanitizeOutgoingCalendarEventData } from "@/lib/calendar-event-normalization";
+import { getEventStartDate } from "@/lib/calendar-utils";
 import { useTaskStore } from "@/stores/task-store";
 import { cn } from "@/lib/utils";
 import type { CalendarEvent, CalendarParticipant } from "@/lib/jmap/types";
@@ -385,12 +386,16 @@ export default function CalendarPage() {
     }
   }, [client, fetchEvents]);
 
-  const focusCalendarOnEvent = useCallback((event: Pick<CalendarEvent, "start">) => {
+  const focusCalendarOnEvent = useCallback((event: Pick<Partial<CalendarEvent>, "start" | "utcStart" | "showWithoutTime">) => {
     if (!event.start) {
       return;
     }
 
-    const eventDate = parseISO(event.start);
+    const eventDate = getEventStartDate({
+      start: event.start,
+      utcStart: event.utcStart ?? null,
+      showWithoutTime: event.showWithoutTime ?? false,
+    });
     if (Number.isNaN(eventDate.getTime())) {
       return;
     }

components/calendar/calendar-agenda-view.tsx

@@ -2,11 +2,11 @@
 
 import { useMemo, useRef, useEffect, useCallback } from "react";
 import { useTranslations, useFormatter } from "next-intl";
-import { format, parseISO, isToday, isTomorrow, startOfDay } from "date-fns";
+import { format, isToday, isTomorrow, startOfDay } from "date-fns";
 import { MapPin, Users } from "lucide-react";
 import { cn } from "@/lib/utils";
 import { parseDuration, getEventColor } from "./event-card";
-import { getEventDayBounds, getPrimaryCalendarId } from "@/lib/calendar-utils";
+import { getEventDayBounds, getEventEndDate, getEventStartDate, getPrimaryCalendarId } from "@/lib/calendar-utils";
 import { getParticipantCount } from "@/lib/calendar-participants";
 import type { CalendarEvent, Calendar } from "@/lib/jmap/types";
 
@@ -49,7 +49,7 @@ export function CalendarAgendaView({
 
   const grouped = useMemo(() => {
     const sorted = [...events].sort((a, b) =>
-      parseISO(a.start).getTime() - parseISO(b.start).getTime()
+      getEventStartDate(a).getTime() - getEventStartDate(b).getTime()
     );
 
     const groups: DayGroup[] = [];
@@ -144,9 +144,9 @@ export function CalendarAgendaView({
               const calId = getPrimaryCalendarId(ev);
               const calendar = calId ? calendarMap.get(calId) : undefined;
               const color = getEventColor(ev, calendar);
-              const start = parseISO(ev.start);
+              const start = getEventStartDate(ev);
               const durMin = parseDuration(ev.duration);
-              const end = new Date(start.getTime() + durMin * 60000);
+              const end = getEventEndDate(ev);
               const locationName = ev.locations
                 ? Object.values(ev.locations)[0]?.name
                 : null;

components/calendar/event-card.tsx

@@ -4,9 +4,10 @@ import { useCallback, useState, type CSSProperties, type DragEvent } from "react
 import { useTranslations } from "next-intl";
 import { cn } from "@/lib/utils";
 import type { CalendarEvent, Calendar } from "@/lib/jmap/types";
-import { format, parseISO } from "date-fns";
+import { format } from "date-fns";
 import { Users } from "lucide-react";
 import { getParticipantCount } from "@/lib/calendar-participants";
+import { getEventEndDate, getEventStartDate } from "@/lib/calendar-utils";
 import { useSettingsStore } from "@/stores/settings-store";
 
 interface EventCardProps {
@@ -68,14 +69,14 @@ export function EventCard({ event, calendar, variant, onClick, onMouseEnter, onM
   const t = useTranslations("calendar");
   const [isBeingDragged, setIsBeingDragged] = useState(false);
   const color = getEventColor(event, calendar);
-  const startDate = parseISO(event.start);
+  const startDate = getEventStartDate(event);
   const timeFormat = useSettingsStore((state) => state.timeFormat);
   const showTimeInMonthView = useSettingsStore((state) => state.showTimeInMonthView);
   const timeFmt = timeFormat === "12h" ? "h:mm a" : "HH:mm";
 
   const calendarName = calendar?.name || "";
   const durationMinutes = parseDuration(event.duration);
-  const endTime = new Date(startDate.getTime() + durationMinutes * 60000);
+  const endTime = getEventEndDate(event);
   const timeString = `${format(startDate, timeFmt)} – ${format(endTime, timeFmt)}`;
   const ariaLabel = `${event.title || t("events.no_title")}, ${timeString}${calendarName ? `, ${calendarName}` : ""}`;
 

components/calendar/event-detail-popover.tsx

@@ -12,6 +12,7 @@ import { format, parseISO } from "date-fns";
 import { cn } from "@/lib/utils";
 import type { CalendarEvent, Calendar, CalendarParticipant } from "@/lib/jmap/types";
 import { parseDuration, getEventColor } from "./event-card";
+import { getEventEndDate, getEventStartDate } from "@/lib/calendar-utils";
 import {
   isOrganizer,
   getUserParticipantId,
@@ -138,9 +139,9 @@ export function EventDetailPopover({
   const [isSavingNote, setIsSavingNote] = useState(false);
 
   const color = getEventColor(event, calendar);
-  const startDate = parseISO(event.start);
+  const startDate = getEventStartDate(event);
   const durationMinutes = parseDuration(event.duration);
-  const endDate = new Date(startDate.getTime() + durationMinutes * 60000);
+  const endDate = getEventEndDate(event);
 
   const locationName = useMemo(() => {
     if (!event.locations) return null;

components/calendar/event-modal.tsx

@@ -8,7 +8,7 @@ import { X, Trash2, Check, Users, CalendarDays, Copy, Pencil, Clock, MapPin, Vid
 import { format, parseISO, addHours, addDays } from "date-fns";
 import type { CalendarEvent, Calendar, CalendarParticipant } from "@/lib/jmap/types";
 import { parseDuration, getEventColor } from "./event-card";
-import { buildAllDayDuration, getEventDisplayEndDate, getPrimaryCalendarId } from "@/lib/calendar-utils";
+import { buildAllDayDuration, getEventDisplayEndDate, getEventEndDate, getEventStartDate, getPrimaryCalendarId } from "@/lib/calendar-utils";
 import { ParticipantInput } from "./participant-input";
 import {
   isOrganizer,
@@ -159,7 +159,7 @@ export function EventModal({
   }, [event, existingParticipants]);
 
   const getInitialStart = (): Date => {
-    if (event?.start) return parseISO(event.start);
+    if (event?.start) return getEventStartDate(event);
     if (defaultDate) {
       const d = new Date(defaultDate);
       if (defaultEndDate) return d;
@@ -177,9 +177,7 @@ export function EventModal({
       if (event.showWithoutTime) {
         return getEventDisplayEndDate(event);
       }
-      const s = parseISO(event.start);
-      const dur = parseDuration(event.duration);
-      return new Date(s.getTime() + dur * 60000);
+      return getEventEndDate(event);
     }
     if (defaultEndDate) return new Date(defaultEndDate);
     return addHours(getInitialStart(), 1);
@@ -384,7 +382,7 @@ export function EventModal({
 
   const handleDuplicate = useCallback(() => {
     if (!event || !onDuplicate) return;
-    const start = parseISO(event.start);
+    const start = getEventStartDate(event);
     const newStart = addDays(start, 1);
     const newUid = typeof crypto !== 'undefined' && crypto.randomUUID
       ? crypto.randomUUID()
@@ -456,9 +454,9 @@ export function EventModal({
   const hasParticipants = attendees.length > 0 || (event?.participants && Object.keys(event.participants).length > 0);
 
   if (isAttendeeMode && event) {
-    const startD = parseISO(event.start);
+    const startD = getEventStartDate(event);
     const durMin = parseDuration(event.duration);
-    const endD = new Date(startD.getTime() + durMin * 60000);
+    const endD = getEventEndDate(event);
     const locationName = event.locations ? Object.values(event.locations)[0]?.name : null;
     const participants = getParticipantList(event);
 
@@ -567,9 +565,9 @@ export function EventModal({
 
   // View mode: read-only display of event details with Edit button
   if (mode === "view" && event) {
-    const startD = parseISO(event.start);
+    const startD = getEventStartDate(event);
     const durMin = parseDuration(event.duration);
-    const endD = new Date(startD.getTime() + durMin * 60000);
+    const endD = getEventEndDate(event);
     const locationName = event.locations ? Object.values(event.locations)[0]?.name || null : null;
     const virtualLoc = event.virtualLocations ? Object.values(event.virtualLocations)[0]?.uri || null : null;
     const viewParticipants = getParticipantList(event);

components/calendar/ical-import-modal.tsx

@@ -7,6 +7,7 @@ import { X, Upload, Check, Loader2, RefreshCw, Globe } from "lucide-react";
 import { format, parseISO } from "date-fns";
 import type { CalendarEvent, Calendar } from "@/lib/jmap/types";
 import type { IJMAPClient } from '@/lib/jmap/client-interface';
+import { getEventStartDate } from "@/lib/calendar-utils";
 import { useCalendarStore } from "@/stores/calendar-store";
 import { useSettingsStore } from "@/stores/settings-store";
 import { toast } from "@/stores/toast-store";
@@ -195,7 +196,7 @@ export function ICalImportModal({ calendars, client, onClose }: ICalImportModalP
   const formatEventDate = (event: Partial<CalendarEvent>): string => {
     if (!event.start) return "";
     try {
-      const date = parseISO(event.start);
+      const date = getEventStartDate(event as CalendarEvent);
       const timeFmt = timeFormat === "12h" ? "h:mm a" : "HH:mm";
       return event.showWithoutTime
         ? format(date, "MMM d, yyyy")

components/calendar/mini-calendar.tsx

@@ -7,9 +7,10 @@ import {
   startOfMonth, endOfMonth, startOfWeek, endOfWeek,
   addMonths, subMonths, addYears, subYears, setMonth, setYear,
   eachDayOfInterval, getMonth, getYear, getISOWeek, getWeek,
-  isSameDay, isSameMonth, isToday, format, parseISO,
+  isSameDay, isSameMonth, isToday, format,
 } from "date-fns";
 import { cn } from "@/lib/utils";
+import { getEventDayBounds } from "@/lib/calendar-utils";
 import type { CalendarEvent } from "@/lib/jmap/types";
 
 type PickerView = "days" | "months" | "years";
@@ -54,7 +55,14 @@ export function MiniCalendar({
   const eventDates = useMemo(() => {
     const set = new Set<string>();
     events.forEach(e => {
-      try { set.add(format(parseISO(e.start), "yyyy-MM-dd")); } catch { /* skip */ }
+      try {
+        const { startDay, endDay } = getEventDayBounds(e);
+        const cursor = new Date(startDay);
+        while (cursor <= endDay) {
+          set.add(format(cursor, "yyyy-MM-dd"));
+          cursor.setDate(cursor.getDate() + 1);
+        }
+      } catch { /* skip */ }
     });
     return set;
   }, [events]);

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);