diff --git a/lib/__tests__/calendar-alerts.test.ts b/lib/__tests__/calendar-alerts.test.ts
index 484ece3a7..c54f5679b 100644
--- a/lib/__tests__/calendar-alerts.test.ts
+++ b/lib/__tests__/calendar-alerts.test.ts
@@ -11,11 +11,13 @@ import {
   alertsToRows,
   rowsToAlerts,
   newAlertRow,
+  computeTaskFireTime,
 } from '../calendar-alerts';
 import type {
   CalendarEvent,
   CalendarEventAlert,
   Calendar,
+  CalendarTask,
 } from '@/lib/jmap/types';
 
 function makeEvent(overrides: Partial<CalendarEvent> = {}): CalendarEvent {
@@ -521,3 +523,26 @@ describe('getPendingAlerts', () => {
     expect(result[0].calendarName).toBeNull();
   });
 });
+
+describe('computeTaskFireTime (#1328)', () => {
+  const task = (start: string | null, due: string | null) => ({ start, due } as CalendarTask);
+  const offset = (relativeTo?: 'start' | 'end') => ({ '@type': 'OffsetTrigger' as const, offset: '-PT5M', ...(relativeTo ? { relativeTo } : {}) }) as Parameters<typeof computeTaskFireTime>[1];
+  const at = (s: string) => new Date(s).getTime();
+
+  it('counts a start-relative alert from the start', () => {
+    expect(computeTaskFireTime(task('2026-10-23T20:00:00Z', '2026-10-23T20:15:00Z'), offset('start'))).toBe(at('2026-10-23T19:55:00Z'));
+  });
+
+  it('treats a missing relativeTo as start, as Stalwart sends it', () => {
+    expect(computeTaskFireTime(task('2026-10-23T20:00:00Z', '2026-10-23T20:15:00Z'), offset())).toBe(at('2026-10-23T19:55:00Z'));
+  });
+
+  it('counts an end-relative alert from the due time', () => {
+    expect(computeTaskFireTime(task('2026-10-23T20:00:00Z', '2026-10-23T20:15:00Z'), offset('end'))).toBe(at('2026-10-23T20:10:00Z'));
+  });
+
+  it('falls back to the due time when the task has no start', () => {
+    expect(computeTaskFireTime(task(null, '2026-10-23T20:15:00Z'), offset('start'))).toBe(at('2026-10-23T20:10:00Z'));
+    expect(computeTaskFireTime(task(null, null), offset('start'))).toBeNull();
+  });
+});
diff --git a/lib/calendar-alerts.ts b/lib/calendar-alerts.ts
index ebcd2d36b..8d6d75cd9 100644
--- a/lib/calendar-alerts.ts
+++ b/lib/calendar-alerts.ts
@@ -275,8 +275,11 @@ export function computeTaskFireTime(
   const offsetMs = parseAlertOffset(trigger.offset);
   if (offsetMs === null) return null;
 
-  if (!task.due) return null;
-  const baseTime = new Date(task.due).getTime();
+  // RFC 8984 §4.5.2: relativeTo defaults to "start", which for a Task is its
+  // start; "end" is its due. A task without a start counts from its due (#1328).
+  const base = trigger.relativeTo !== 'end' && task.start ? task.start : task.due;
+  if (!base) return null;
+  const baseTime = new Date(base).getTime();
   if (Number.isNaN(baseTime)) return null;
   return baseTime + offsetMs;
 }