calendar: read all-day events with end == start as one day

integrations/calendar/adapter.test.ts

@@ -125,11 +125,20 @@ describe('Google Calendar package', () => {
       end: '2026-09-23',
       allDay: true,
     });
-    expect(() =>
+    // end == start, as Google Calendar shows it: one day, exclusive end.
+    expect(
       project(
         timed('e3', {
+          start: { date: '2026-12-31' },
+          end: { date: '2026-12-31' },
+        }),
+      ),
+    ).toMatchObject({ start: '2026-12-31', end: '2027-01-01', allDay: true });
+    expect(() =>
+      project(
+        timed('e3b', {
           start: { date: '2026-09-22' },
-          end: { date: '2026-09-22' },
+          end: { date: '2026-09-21' },
         }),
       ),
     ).toThrow('invalid all-day interval');
@@ -139,11 +148,11 @@ describe('Google Calendar package', () => {
   });
 
   it('skips and lists an unreadable event instead of failing the whole scan', async () => {
-    // Seen in user testing (2026-09-28): one all-day event failed every sync.
+    // In user testing (2026-09-28) one odd all-day event failed every sync.
     const bad = timed('bad', {
       summary: 'testing',
       start: { date: '2026-04-02' },
-      end: { date: '2026-04-02' },
+      end: { date: '2026-04-01' },
     });
     const result = await preview(
       {
@@ -185,7 +194,7 @@ describe('Google Calendar package', () => {
         id: 'bad',
         title: 'testing',
         reason:
-          'Calendar event bad has an invalid all-day interval (start "2026-04-02", end "2026-04-02")',
+          'Calendar event bad has an invalid all-day interval (start "2026-04-02", end "2026-04-01")',
       },
     ]);
     // Its existing row is kept: a conflict, never an inferred deletion.

integrations/calendar/adapter.ts

@@ -113,6 +113,18 @@ function civilDate(value: string): boolean {
   );
 }
 
+/** Google's own writes give an all-day event an exclusive end, the day after
+ * its last day. Events with end == start exist too (seen in user testing,
+ * 2026-09-28: all-day events written with an inclusive end), and Google
+ * Calendar shows them as one day, so they are read as that one day. Any
+ * write-back then sends the exclusive end. */
+function allDayEnd(start: string, end: string): string {
+  if (end !== start) return end;
+  const next = new Date(Date.parse(`${start}T00:00:00Z`) + 86_400_000);
+
+  return next.toISOString().slice(0, 10);
+}
+
 function offsetDateTime(value: string): boolean {
   return (
     /^\d{4}-\d{2}-\d{2}T.*(?:Z|[+-]\d{2}:\d{2})$/.test(value) &&
@@ -146,7 +158,7 @@ export function project(event: Event): Projection | undefined {
       !civilDate(event.start.date) ||
       !event.end.date ||
       !civilDate(event.end.date) ||
-      event.end.date <= event.start.date
+      event.end.date < event.start.date
     )
       throw new UnreadableEventError(
         `Calendar event ${event.id} has an invalid all-day interval (${interval()})`,
@@ -171,7 +183,9 @@ export function project(event: Event): Projection | undefined {
     description: event.description ?? '',
     location: event.location ?? '',
     start: allDay ? event.start.date! : event.start.dateTime!,
-    end: allDay ? event.end.date! : event.end.dateTime!,
+    end: allDay
+      ? allDayEnd(event.start.date!, event.end.date!)
+      : event.end.dateTime!,
     allDay,
   };
 }