From 4de856923b7a5cb0ad7fd109548834458ad00d0f Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Thu, 29 Oct 2026 03:18:00 +0100
Subject: [PATCH] fix: keep daily recurring events going past a DST gap

---
 src/lib/__tests__/recurrence-dst-gap.test.ts | 40 ++++++++++++++++++++
 src/lib/recurrence-expansion.ts              | 23 ++++++++---
 2 files changed, 58 insertions(+), 5 deletions(-)
 create mode 100644 src/lib/__tests__/recurrence-dst-gap.test.ts

diff --git a/src/lib/__tests__/recurrence-dst-gap.test.ts b/src/lib/__tests__/recurrence-dst-gap.test.ts
new file mode 100644
index 00000000..e568091e
--- /dev/null
+++ b/src/lib/__tests__/recurrence-dst-gap.test.ts
@@ -0,0 +1,40 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import type { CalendarEvent } from '../../api/types';
+import { expandRecurringEvents } from '../recurrence-expansion';
+
+// Daily expansion advanced a Date across the device's DST gap and kept the
+// shifted time (02:30 became 03:30, midnight became 01:00); the implicit
+// byHour then rejected every later day, so the series ended at the gap.
+
+function daily(start: string, allDay: boolean, timeZone?: string): CalendarEvent {
+  return {
+    id: 'E1', uid: 'u1', title: 'Daily', calendarIds: { c: true },
+    start, duration: allDay ? 'P1D' : 'PT30M', showWithoutTime: allDay,
+    ...(timeZone ? { timeZone } : {}),
+    recurrenceRules: [{ '@type': 'RecurrenceRule', frequency: 'daily' }],
+  } as unknown as CalendarEvent;
+}
+
+let originalTZ: string | undefined;
+beforeEach(() => {
+  originalTZ = process.env.TZ;
+});
+afterEach(() => {
+  process.env.TZ = originalTZ;
+});
+
+describe('daily recurrence across a DST gap', () => {
+  it('keeps a 02:30 series going past the spring-forward night (Berlin)', () => {
+    process.env.TZ = 'Europe/Berlin';
+    const out = expandRecurringEvents([daily('2026-03-01T02:30:00', false, 'Europe/Berlin')], '2026-03-01T00:00:00', '2026-04-01T00:00:00');
+    expect(out).toHaveLength(31);
+    expect(out.at(-1)!.start.slice(0, 16)).toBe('2026-03-31T02:30');
+  });
+
+  it('keeps an all-day series going after a midnight gap (Santiago)', () => {
+    process.env.TZ = 'America/Santiago';
+    const out = expandRecurringEvents([daily('2026-08-20T00:00:00', true)], '2026-09-07T00:00:00', '2026-09-14T00:00:00');
+    expect(out).toHaveLength(7);
+    expect(out.map((o) => o.start.slice(11, 16))).toEqual(Array(7).fill('00:00'));
+  });
+});
diff --git a/src/lib/recurrence-expansion.ts b/src/lib/recurrence-expansion.ts
index cdef6eba..fae990ec 100644
--- a/src/lib/recurrence-expansion.ts
+++ b/src/lib/recurrence-expansion.ts
@@ -524,6 +524,16 @@ function generateCandidatesForPeriod(
       candidates = expandWeekly(periodStart, rule, eventStart);
       break;
     case 'daily':
+      // The start's wall-clock time on this day. The period anchor is
+      // advanced day by day and keeps whatever time a DST gap pushed it to
+      // (02:30 -> 03:30, or midnight -> 01:00 for an all-day event), so using
+      // it made every later day fail the implicit byHour and the series
+      // stopped at the gap. Mirrors the webmail (34fb9d247).
+      candidates = [new Date(
+        periodStart.getFullYear(), periodStart.getMonth(), periodStart.getDate(),
+        eventStart.getHours(), eventStart.getMinutes(), eventStart.getSeconds(), eventStart.getMilliseconds(),
+      )];
+      break;
     case 'hourly':
     case 'minutely':
     case 'secondly':
@@ -532,7 +542,10 @@ function generateCandidatesForPeriod(
       break;
   }
 
-  candidates = candidates.filter((d) => matchesByX(d, rule));
+  // A daily candidate is judged by the time it was built for: on the day of
+  // a DST gap that time does not exist and the occurrence moves past the gap
+  // (RFC 5545 3.3.5), which is still this occurrence.
+  candidates = candidates.filter((d) => matchesByX(d, rule, freq === 'daily' ? eventStart : undefined));
   candidates.sort((a, b) => a.getTime() - b.getTime());
   return candidates;
 }
@@ -660,7 +673,7 @@ function expandWeekly(
   return dates.sort((a, b) => a.getTime() - b.getTime());
 }
 
-function matchesByX(date: Date, rule: RecurrenceRule): boolean {
+function matchesByX(date: Date, rule: RecurrenceRule, timeOf: Date = date): boolean {
   if (rule.byMonth?.length) {
     const month = String(date.getMonth() + 1);
     if (!rule.byMonth.some((m) => m.replace('L', '') === month)) return false;
@@ -706,9 +719,9 @@ function matchesByX(date: Date, rule: RecurrenceRule): boolean {
       })
     ) return false;
   }
-  if (rule.byHour?.length && !rule.byHour.includes(date.getHours())) return false;
-  if (rule.byMinute?.length && !rule.byMinute.includes(date.getMinutes())) return false;
-  if (rule.bySecond?.length && !rule.bySecond.includes(date.getSeconds())) return false;
+  if (rule.byHour?.length && !rule.byHour.includes(timeOf.getHours())) return false;
+  if (rule.byMinute?.length && !rule.byMinute.includes(timeOf.getMinutes())) return false;
+  if (rule.bySecond?.length && !rule.bySecond.includes(timeOf.getSeconds())) return false;
   return true;
 }