fix: proper support for all-day events

components/email/calendar-invitation-banner.tsx

@@ -544,6 +544,7 @@ export function CalendarInvitationBanner({ email }: CalendarInvitationBannerProp
           dtStart: parsedEvent.start || undefined,
           dtEnd: summary?.end || undefined,
           timeZone: parsedEvent.timeZone || undefined,
+          isAllDay: parsedEvent.showWithoutTime || false,
           sequence: parsedEvent.sequence,
           status: imipStatus,
         });
@@ -659,10 +660,19 @@ export function CalendarInvitationBanner({ email }: CalendarInvitationBannerProp
     }
   };
 
+  const isAllDayEvent = parsedEvent?.showWithoutTime ?? false;
+
   const formatDateTime = (dateStr: string | null) => {
     if (!dateStr) return '';
     const date = new Date(dateStr);
     if (isNaN(date.getTime())) return dateStr;
+    if (isAllDayEvent) {
+      return format.dateTime(date, {
+        weekday: 'short',
+        month: 'short',
+        day: 'numeric',
+      });
+    }
     return format.dateTime(date, {
       weekday: 'short',
       month: 'short',

lib/__tests__/calendar-invitation.test.ts

@@ -335,10 +335,7 @@ describe('formatEventSummary', () => {
       duration: 'PT1H30M',
     });
     expect(summary.start).toBe('2026-02-17T10:00:00');
-    expect(summary.end).toBeTruthy();
-    const endDate = new Date(summary.end!);
-    expect(endDate.getHours()).toBe(new Date('2026-02-17T10:00:00').getHours() + 1);
-    expect(endDate.getMinutes()).toBe(new Date('2026-02-17T10:00:00').getMinutes() + 30);
+    expect(summary.end).toBe('2026-02-17T11:30:00');
   });
 
   it('uses utcStart and utcEnd when available', () => {
@@ -349,6 +346,38 @@ describe('formatEventSummary', () => {
     });
     expect(summary.start).toBe('2026-02-17T15:00:00Z');
     expect(summary.end).toBe('2026-02-17T16:00:00Z');
+    expect(summary.isAllDay).toBe(false);
+  });
+
+  it('prefers local start over utcStart for all-day events', () => {
+    const summary = formatEventSummary({
+      utcStart: '2026-03-16T00:00:00Z',
+      utcEnd: '2026-03-17T00:00:00Z',
+      start: '2026-03-16T00:00:00',
+      showWithoutTime: true,
+    });
+    expect(summary.start).toBe('2026-03-16T00:00:00');
+    expect(summary.end).toBe('2026-03-17T00:00:00Z');
+    expect(summary.isAllDay).toBe(true);
+  });
+
+  it('computes all-day end in local format when utcEnd is missing', () => {
+    const summary = formatEventSummary({
+      start: '2026-03-16T00:00:00',
+      duration: 'P1D',
+      showWithoutTime: true,
+    });
+    expect(summary.start).toBe('2026-03-16T00:00:00');
+    expect(summary.end).toBe('2026-03-17T00:00:00');
+    expect(summary.isAllDay).toBe(true);
+  });
+
+  it('returns local format end for timed events with local start', () => {
+    const summary = formatEventSummary({
+      start: '2026-02-17T10:00:00',
+      duration: 'PT1H30M',
+    });
+    expect(summary.end).toBe('2026-02-17T11:30:00');
   });
 });
 

lib/calendar-invitation.ts

@@ -449,6 +449,7 @@ export interface EventSummary {
   title: string;
   start: string | null;
   end: string | null;
+  isAllDay: boolean;
   location: string | null;
   organizer: string | null;
   organizerEmail: string | null;
@@ -495,6 +496,8 @@ export function formatEventSummary(event: Partial<CalendarEvent>): EventSummary
     if (!organizer) organizer = organizerEmail;
   }
 
+  const isAllDay = event.showWithoutTime ?? false;
+
   let end: string | null = null;
   if (event.utcEnd) {
     end = event.utcEnd;
@@ -502,10 +505,17 @@ export function formatEventSummary(event: Partial<CalendarEvent>): EventSummary
     end = addDurationToDate(event.start, event.duration, event.timeZone);
   }
 
+  // For all-day events, prefer the local start (no timezone) to avoid
+  // UTC conversion shifting the displayed date in non-UTC timezones.
+  const start = isAllDay
+    ? (event.start || null)
+    : (event.utcStart || event.start || null);
+
   return {
     title: event.title || '',
-    start: event.utcStart || event.start || null,
+    start,
     end,
+    isAllDay,
     location,
     organizer,
     organizerEmail,
@@ -530,6 +540,19 @@ function addDurationToDate(start: string, duration: string, _timeZone?: string |
   date.setMinutes(date.getMinutes() + minutes);
   date.setSeconds(date.getSeconds() + seconds);
 
+  // If the input is a local datetime (no UTC 'Z' suffix), return a local
+  // format string so that all-day date arithmetic isn't shifted by the
+  // browser's UTC offset (toISOString converts to UTC).
+  if (!start.endsWith('Z') && !start.includes('+')) {
+    const y = date.getFullYear();
+    const m = String(date.getMonth() + 1).padStart(2, '0');
+    const d = String(date.getDate()).padStart(2, '0');
+    const h = String(date.getHours()).padStart(2, '0');
+    const min = String(date.getMinutes()).padStart(2, '0');
+    const s = String(date.getSeconds()).padStart(2, '0');
+    return `${y}-${m}-${d}T${h}:${min}:${s}`;
+  }
+
   return date.toISOString();
 }
 

lib/jmap/client.ts

@@ -1490,6 +1490,7 @@ export class JMAPClient {
     dtStart?: string;
     dtEnd?: string;
     timeZone?: string;
+    isAllDay?: boolean;
     sequence?: number;
     status: 'ACCEPTED' | 'TENTATIVE' | 'DECLINED';
     identityId?: string;
@@ -1542,20 +1543,30 @@ export class JMAPClient {
       `DTSTAMP:${now}`,
     ];
     if (opts.dtStart) {
-      const formatted = formatIcalDate(opts.dtStart, opts.timeZone);
-      // If TZID is included, it's a parameter on the property
-      if (formatted.startsWith('TZID=')) {
-        lines.push(`DTSTART;${formatted}`);
+      if (opts.isAllDay) {
+        // RFC 5545 §3.3.4: all-day events use VALUE=DATE (date-only, no time)
+        const dateOnly = opts.dtStart.replace(/[-]/g, '').substring(0, 8);
+        lines.push(`DTSTART;VALUE=DATE:${dateOnly}`);
       } else {
-        lines.push(`DTSTART:${formatted}`);
+        const formatted = formatIcalDate(opts.dtStart, opts.timeZone);
+        if (formatted.startsWith('TZID=')) {
+          lines.push(`DTSTART;${formatted}`);
+        } else {
+          lines.push(`DTSTART:${formatted}`);
+        }
       }
     }
     if (opts.dtEnd) {
-      const formatted = formatIcalDate(opts.dtEnd, opts.timeZone);
-      if (formatted.startsWith('TZID=')) {
-        lines.push(`DTEND;${formatted}`);
+      if (opts.isAllDay) {
+        const dateOnly = opts.dtEnd.replace(/[-]/g, '').substring(0, 8);
+        lines.push(`DTEND;VALUE=DATE:${dateOnly}`);
       } else {
-        lines.push(`DTEND:${formatted}`);
+        const formatted = formatIcalDate(opts.dtEnd, opts.timeZone);
+        if (formatted.startsWith('TZID=')) {
+          lines.push(`DTEND;${formatted}`);
+        } else {
+          lines.push(`DTEND:${formatted}`);
+        }
       }
     }
     if (opts.summary) {