From f358d60584232a754beceaca5dfce0f80055b071 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Sun, 25 Oct 2026 14:18:45 +0100
Subject: [PATCH] fix: place calendar events at their clock time on DST change
 days

---
 src/lib/__tests__/calendar-utils.test.ts | 35 ++++++++++++++++++++++++
 src/lib/calendar-utils.ts                | 28 ++++++++++++-------
 2 files changed, 53 insertions(+), 10 deletions(-)

diff --git a/src/lib/__tests__/calendar-utils.test.ts b/src/lib/__tests__/calendar-utils.test.ts
index 5c1b0a66..16e8db3d 100644
--- a/src/lib/__tests__/calendar-utils.test.ts
+++ b/src/lib/__tests__/calendar-utils.test.ts
@@ -14,6 +14,7 @@ import {
   layoutOverlappingEvents,
   CALENDAR_COLOR_PALETTE,
   reopenedEdit,
+  getTimedEventBoundsForDay,
 } from '../calendar-utils';
 import type { CalendarEvent } from '../../api/types';
 import { useSettingsStore } from '../../stores/settings-store';
@@ -294,3 +295,37 @@ describe('reopenedEdit', () => {
     expect('locations' in reopened).toBe(false);
   });
 });
+
+describe('getTimedEventBoundsForDay on DST change days', () => {
+  it('places events by wall-clock time, not elapsed time since midnight', () => {
+    const tz = process.env.TZ;
+    process.env.TZ = 'Europe/Berlin';
+    try {
+      // 2026-10-25 has 25 hours in Berlin; 12:51 CET is 13h51m after midnight.
+      const fallBack = ev({
+        start: '2026-10-25T12:51:00',
+        timeZone: 'Europe/Berlin',
+        utcStart: '2026-10-25T11:51:00Z',
+        utcEnd: '2026-10-25T12:16:00Z',
+      });
+      expect(getTimedEventBoundsForDay(fallBack, new Date(2026, 9, 25))).toMatchObject({
+        startMinutes: 12 * 60 + 51,
+        endMinutes: 13 * 60 + 16,
+      });
+
+      // 2026-03-29 has 23 hours; 09:00 CEST is 8h after midnight.
+      const springForward = ev({
+        start: '2026-03-29T09:00:00',
+        timeZone: 'Europe/Berlin',
+        utcStart: '2026-03-29T07:00:00Z',
+        utcEnd: '2026-03-29T08:00:00Z',
+      });
+      expect(getTimedEventBoundsForDay(springForward, new Date(2026, 2, 29))).toMatchObject({
+        startMinutes: 9 * 60,
+        endMinutes: 10 * 60,
+      });
+    } finally {
+      process.env.TZ = tz;
+    }
+  });
+});
diff --git a/src/lib/calendar-utils.ts b/src/lib/calendar-utils.ts
index d1cf1c94..b39a82c6 100644
--- a/src/lib/calendar-utils.ts
+++ b/src/lib/calendar-utils.ts
@@ -263,6 +263,15 @@ export interface TimedDayBounds {
   continuesAfter: boolean;
 }
 
+function wallClockMinutes(date: Date): number {
+  return (
+    date.getHours() * 60 +
+    date.getMinutes() +
+    date.getSeconds() / 60 +
+    date.getMilliseconds() / 60000
+  );
+}
+
 export function getTimedEventBoundsForDay(
   event: CalendarEvent,
   day: Date,
@@ -276,16 +285,15 @@ export function getTimedEventBoundsForDay(
 
   if (eventEnd <= dayStart || eventStart >= nextDayStart) return null;
 
-  const clippedStart = eventStart > dayStart ? eventStart : dayStart;
-  const clippedEnd = eventEnd < nextDayStart ? eventEnd : nextDayStart;
-  const startMinutes = Math.max(
-    0,
-    Math.floor((clippedStart.getTime() - dayStart.getTime()) / 60000),
-  );
-  const endMinutes = Math.min(
-    1440,
-    Math.ceil((clippedEnd.getTime() - dayStart.getTime()) / 60000),
-  );
+  // Wall-clock minutes, not elapsed time since midnight: on a DST change day
+  // the day has 23 or 25 hours, and elapsed time would shift every event
+  // after the change by an hour.
+  const startMinutes =
+    eventStart > dayStart ? Math.floor(wallClockMinutes(eventStart)) : 0;
+  const endMinutes =
+    eventEnd < nextDayStart
+      ? Math.max(startMinutes, Math.ceil(wallClockMinutes(eventEnd)))
+      : 1440;
 
   return {
     startMinutes,