From 6f475c0c52751c3712a513be233dc8a1fea92250 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Thu, 29 Oct 2026 03:14:58 +0100
Subject: [PATCH] fix: remind of a task relative to its start when the reminder
 says so #83

---
 .../calendar-alert-scheduler.test.ts          | 31 +++++++++++++++++--
 src/lib/calendar-alert-scheduler.ts           | 10 ++++++
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/src/lib/__tests__/calendar-alert-scheduler.test.ts b/src/lib/__tests__/calendar-alert-scheduler.test.ts
index 6147cf08..9a5c7864 100644
--- a/src/lib/__tests__/calendar-alert-scheduler.test.ts
+++ b/src/lib/__tests__/calendar-alert-scheduler.test.ts
@@ -49,8 +49,8 @@ describe('parseAlertOffset / computeFireTime', () => {
     );
   });
 
-  it('fires task alerts relative to due', () => {
-    const task = { ...event, due: '2026-03-02T09:00:00', alerts: undefined };
+  it('fires the alerts of a task without a start relative to its due', () => {
+    const task = { ...event, start: undefined as unknown as string, utcStart: undefined, due: '2026-03-02T09:00:00', alerts: undefined };
     expect(computeTaskFireTime(task, { '@type': 'OffsetTrigger', offset: '-PT1H' })).toBe(
       new Date('2026-03-02T09:00:00').getTime() - HOUR,
     );
@@ -58,11 +58,32 @@ describe('parseAlertOffset / computeFireTime', () => {
   });
 
   it('reads a zoned task due in its own time zone', () => {
-    const task = { ...event, due: '2026-07-01T17:00:00', timeZone: 'Europe/Berlin', alerts: undefined };
+    const task = { ...event, start: undefined as unknown as string, utcStart: undefined, due: '2026-07-01T17:00:00', timeZone: 'Europe/Berlin', alerts: undefined };
     expect(computeTaskFireTime(task, { '@type': 'OffsetTrigger', offset: '-PT1H' })).toBe(
       new Date('2026-07-01T14:00:00Z').getTime(),
     );
   });
+
+  // #83 (webmail #1328): start 20:00, due 20:15, five minutes before the start.
+  it('fires a task alert relative to its start from the start, and one relative to its end from the due', () => {
+    const task = {
+      ...event, start: '2026-07-01T20:00:00', utcStart: undefined, due: '2026-07-01T20:15:00', timeZone: 'Europe/Berlin', alerts: undefined,
+    };
+    expect(computeTaskFireTime(task, { '@type': 'OffsetTrigger', offset: '-PT5M', relativeTo: 'start' })).toBe(
+      new Date('2026-07-01T17:55:00Z').getTime(),
+    );
+    // relativeTo left out means the start.
+    expect(computeTaskFireTime(task, { '@type': 'OffsetTrigger', offset: '-PT5M' })).toBe(
+      new Date('2026-07-01T17:55:00Z').getTime(),
+    );
+    expect(computeTaskFireTime(task, { '@type': 'OffsetTrigger', offset: '-PT5M', relativeTo: 'end' })).toBe(
+      new Date('2026-07-01T18:10:00Z').getTime(),
+    );
+    // The server's UTC start, when it gave one.
+    expect(computeTaskFireTime({ ...task, utcStart: '2026-07-01T18:00:00Z' }, { '@type': 'OffsetTrigger', offset: '-PT5M' })).toBe(
+      new Date('2026-07-01T17:55:00Z').getTime(),
+    );
+  });
 });
 
 describe('getEffectiveAlerts', () => {
@@ -91,6 +112,8 @@ describe('getUpcomingAlerts', () => {
     const task = {
       ...event,
       id: 't1',
+      start: undefined as unknown as string,
+      utcStart: undefined,
       due: '2026-03-01T12:00:00Z',
       alerts: { x: { trigger: { '@type': 'OffsetTrigger', offset: 'PT0S' }, action: 'display' } },
     };
@@ -109,6 +132,8 @@ describe('getUpcomingAlerts', () => {
     const task = {
       ...untitled,
       id: 't1',
+      start: undefined as unknown as string,
+      utcStart: undefined,
       due: '2026-03-01T12:00:00Z',
       alerts: { x: { trigger: { '@type': 'OffsetTrigger' as const, offset: 'PT0S' }, action: 'display' as const } },
     };
diff --git a/src/lib/calendar-alert-scheduler.ts b/src/lib/calendar-alert-scheduler.ts
index a224e7c4..c02345f4 100644
--- a/src/lib/calendar-alert-scheduler.ts
+++ b/src/lib/calendar-alert-scheduler.ts
@@ -83,6 +83,16 @@ export function computeTaskFireTime(task: CalendarEvent, trigger: Alert['trigger
   if (trigger.offset === undefined) return null;
   const offsetMs = parseAlertOffset(trigger.offset);
   if (offsetMs === null) return null;
+  // 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
+  // (#83, webmail #1328).
+  if (trigger.relativeTo !== 'end' && task.start) {
+    const start = task.utcStart
+      ? new Date(task.utcStart)
+      // The start read like a due: in the task's zone, or a calendar date.
+      : getTaskDueDate({ due: task.start, timeZone: task.timeZone, showWithoutTime: task.showWithoutTime });
+    if (start && !Number.isNaN(start.getTime())) return start.getTime() + offsetMs;
+  }
   const due = getTaskDueDate(task);
   if (!due) return null;
   return due.getTime() + offsetMs;