fix: fix bug

Author: sjbur

packages/devextreme/js/__internal/scheduler/m_utils_time_zone.ts

@@ -258,6 +258,18 @@ const isLocalTimeMidnightDST = (date: Date): boolean => {
   return startDayDate.getHours() === 1;
 };
 
+const adjustDayIntervalMinForMidnightDST = (
+  dayIntervalMin: number,
+  startDayHour: number,
+): number => {
+  const date = createDateFromUTCWithLocalOffset(new Date(dayIntervalMin));
+  const isMidnightDST = startDayHour === 0 && isLocalTimeMidnightDST(date);
+
+  return isMidnightDST
+    ? dayIntervalMin + date.getHours() * 60 * 60 * 1000
+    : dayIntervalMin;
+};
+
 const utils = {
   getDaylightOffset,
   getDaylightOffsetInMs,
@@ -285,6 +297,7 @@ const utils = {
   cacheTimeZones,
 
   isLocalTimeMidnightDST,
+  adjustDayIntervalMinForMidnightDST,
 };
 
 export default utils;

packages/devextreme/js/__internal/scheduler/view_model/common/trim_interval.ts

@@ -1,5 +1,10 @@
 import type { DateInterval } from '../types';
 
+export const clampIntervalMin = (interval: DateInterval, minFloor: number): DateInterval => ({
+  ...interval,
+  min: Math.max(interval.min, minFloor),
+});
+
 export const trimInterval = ({
   min,
   max,

packages/devextreme/js/__internal/scheduler/view_model/generate_view_model/options/get_minutes_cell_intervals.ts

@@ -1,3 +1,4 @@
+import timeZoneUtils from '../../../m_utils_time_zone';
 import { splitIntervalByDay } from '../../common/split_interval_by_days';
 import type { CellInterval, DateInterval } from '../../types';
 
@@ -30,8 +31,12 @@ export const getMinutesCellIntervals = ({
 
   let columnIndex = 0;
   filterBySkippedDays(dayIntervals, skippedDays).forEach((dayInterval) => {
-    const date = new Date(dayInterval.min);
-    while (date.getTime() < dayInterval.max) {
+    const firstAvailableDayTime = timeZoneUtils
+      .adjustDayIntervalMinForMidnightDST(dayInterval.min, startDayHour);
+    const intervalMin = Math.max(firstAvailableDayTime, interval.min);
+    const intervalMax = Math.min(dayInterval.max, interval.max);
+    const date = new Date(intervalMin);
+    while (date.getTime() < intervalMax) {
       const min = date.getTime();
       let max = date.setUTCMinutes(date.getUTCMinutes() + durationMinutes);
 

packages/devextreme/js/__internal/scheduler/view_model/generate_view_model/options/get_week_intervals.cairo.test.ts

@@ -0,0 +1,48 @@
+/**
+ * @timezone Africa/Cairo
+ */
+
+import {
+  describe, expect, it,
+} from '@jest/globals';
+
+import { getWeekIntervals } from './get_week_intervals';
+
+describe('getWeekIntervals', () => {
+  it('T1327394: should not add a timeline cell before the first available local time on midnight DST', () => {
+    const intervals = getWeekIntervals({
+      startDayHour: 0,
+      endDayHour: 24,
+      min: Date.UTC(2026, 3, 24, 1),
+      max: Date.UTC(2026, 3, 25),
+      skippedDays: [],
+    }, 60, 0, true);
+
+    expect(intervals.cells[0]).toEqual({
+      min: Date.UTC(2026, 3, 24, 1),
+      max: Date.UTC(2026, 3, 24, 2),
+      rowIndex: 0,
+      columnIndex: 0,
+      cellIndex: 0,
+    });
+  });
+
+  it('T1327394: should not add a timeline week cell before the first available local time on midnight DST', () => {
+    const intervals = getWeekIntervals({
+      startDayHour: 0,
+      endDayHour: 24,
+      min: Date.UTC(2026, 3, 20),
+      max: Date.UTC(2026, 3, 27),
+      skippedDays: [],
+    }, 60, 0, true);
+
+    expect(intervals.cells.find((cell) => cell.min === Date.UTC(2026, 3, 24))).toBeUndefined();
+    expect(intervals.cells.find((cell) => cell.min === Date.UTC(2026, 3, 24, 1))).toEqual({
+      min: Date.UTC(2026, 3, 24, 1),
+      max: Date.UTC(2026, 3, 24, 2),
+      rowIndex: 0,
+      columnIndex: 96,
+      cellIndex: 96,
+    });
+  });
+});

packages/devextreme/js/__internal/scheduler/view_model/generate_view_model/options/get_week_intervals.ts

@@ -1,6 +1,6 @@
 import { shiftIntervals } from '../../common/shift_intervals';
 import { splitIntervalByDay } from '../../common/split_interval_by_days';
-import { trimInterval } from '../../common/trim_interval';
+import { clampIntervalMin, trimInterval } from '../../common/trim_interval';
 import type { CompareOptions, LayoutIntervals } from '../../types';
 import { getMinutesCellIntervals } from './get_minutes_cell_intervals';
 
@@ -14,12 +14,15 @@ export const getWeekIntervals = (
   const trimmedInterval = trimInterval(dateInterval);
   const splitIntervals = splitIntervalByDay(compareOptions);
   const intervals = isTimeline ? [trimmedInterval] : splitIntervals;
+  const cellIntervals = isTimeline
+    ? [clampIntervalMin(trimmedInterval, dateInterval.min)]
+    : intervals;
   const shiftedIntervals = shiftIntervals(intervals, viewOffset);
   const shiftedSplitIntervals = shiftIntervals(splitIntervals, viewOffset);
 
   const cells = getMinutesCellIntervals({
     ...compareOptions,
-    intervals,
+    intervals: cellIntervals,
     durationMinutes: cellDurationMinutes,
   });
   const shiftedCells = shiftIntervals(cells, viewOffset);