Fix DST change causing getWeekViewEventSpan to return wrong span

src/calendar-utils.ts

@@ -46,10 +46,10 @@
   }: {
     event: CalendarEvent;
     sourceEvent: MouseEvent | KeyboardEvent;
   }): any;
 }
 
 export interface CalendarEvent<MetaType = any> {
   id?: string | number;
   start: Date;
   end?: Date;
@@ -85,7 +85,7 @@
   hourColumns: WeekViewHourColumn[];
 }
 
 export interface MonthViewDay<MetaType = any> extends WeekDay {
   inMonth: boolean;
   events: CalendarEvent[];
   backgroundColor?: string;
@@ -158,7 +158,13 @@
     return 0;
   }
   const { addSeconds, getDay, addDays } = dateAdapter;
-  const endDate: Date = addSeconds(startDate, seconds - 1);
+  let endDate = addSeconds(startDate, seconds - 1);
+  if (precision === 'days') {
+    // otherwise when changing to DST hour may shift before the hour of the start date
+    const daysDiff = Math.round(seconds / SECONDS_IN_DAY);
+    endDate = addSeconds(addDays(startDate, daysDiff), -1);
+  }
+
   const dayStart: number = getDay(startDate);
   const dayEnd: number = getDay(endDate);
   let result = 0; // Calculated in seconds
@@ -232,13 +238,8 @@
     totalDaysInView: number;
   }
 ): number {
-  const {
-    max,
-    differenceInSeconds,
-    addDays,
-    endOfDay,
-    differenceInDays,
-  } = dateAdapter;
+  const { max, differenceInSeconds, addDays, endOfDay, differenceInDays } =
+    dateAdapter;
   let span: number = SECONDS_IN_DAY;
   const begin: Date = max([event.start, startOfWeekDate]);
 
@@ -1175,7 +1176,7 @@
 
 export function validateEvents(
   events: CalendarEvent[],
   log: (...args: any[]) => void
 ): boolean {
   let isValid = true;
 

test/calendar-utils-dst.spec.ts

@@ -1,9 +1,11 @@
 import './util/use-london-timezone';
-import { getWeekView } from '../src/calendar-utils';
+import { DEFAULT_TIMEZONE } from './util/use-london-timezone';
+import { getWeekView, getAllDayWeekEvents } from '../src/calendar-utils';
 import { adapterFactory as dateFnsAdapterFactory } from '../src/date-adapters/date-fns';
 import { adapterFactory as momentAdapterFactory } from '../src/date-adapters/moment';
 import { adapterFactory as luxonAdapterFactory } from '../src/date-adapters/luxon';
-import { startOfDay } from 'date-fns';
+import { startOfDay, setHours, setMinutes } from 'date-fns';
+import { register } from 'timezone-mock';
 import * as moment from 'moment';
 import * as luxon from 'luxon';
 
@@ -71,3 +73,45 @@ describe('getTimezoneOffset', () => {
     expect(luxonOffset).toBe(-60);
   });
 });
+
+describe('getAllDayWeekEvents', () => {
+  beforeEach(() => {
+    // Mock Date/date-fns (uses native Date)
+    register('US/Eastern'); // Toronto uses Eastern timezone
+  });
+
+  afterEach(() => {
+    register(DEFAULT_TIMEZONE); // Restore default timezone for other tests
+  });
+
+  it('should correctly position an all-day event spanning from Friday before the DST change to the next Friday (Toronto) with excluded days [0, 6]', () => {
+    // View: March 3–15, 2026 so we see both Fridays (DST springs forward in Toronto on March 9)
+    const viewStart = new Date('2026-03-03');
+    const viewEnd = new Date('2026-03-15');
+
+    // Event spans Fri Mar 7 (before DST) – Fri Mar 14 (after DST), crossing the timezone change weekend
+    const event = {
+      title: 'Week spanning DST',
+      start: setMinutes(setHours(new Date('2026-03-07'), 8), 0),
+      end: setMinutes(setHours(new Date('2026-03-14'), 8), 0),
+      allDay: true,
+    };
+
+    const result = getAllDayWeekEvents(dateAdapter, {
+      viewStart,
+      viewEnd,
+      excluded: [0, 6], // Exclude Sunday and Saturday
+      events: [event],
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].row).toHaveLength(1);
+    // With excluded [0, 6], visible days are Mon 3, Tue 4, Wed 5, Thu 6, Fri 7, Mon 10, Tue 11, Wed 12, Thu 13, Fri 14
+    // Event spans Fri 7 → Fri 14 → offset 4 (Fri 7), span 6 (Fri 7, Mon 10, Tue 11, Wed 12, Thu 13, Fri 14)
+    expect(result[0].row[0].offset).toBe(4);
+    expect(result[0].row[0].span).toBe(6);
+    expect(result[0].row[0].startsBeforeWeek).toBe(false);
+    expect(result[0].row[0].endsAfterWeek).toBe(false);
+    expect(result[0].row[0].event).toEqual(event);
+  });
+});

test/util/use-london-timezone.ts

@@ -1,3 +1,4 @@
 import { register } from 'timezone-mock';
 
-register('Europe/London');
+export const DEFAULT_TIMEZONE = 'Europe/London';
+register(DEFAULT_TIMEZONE);