Draft - Fix the T1255474 bug

packages/devextreme/js/__internal/scheduler/appointments/m_settings_generator.ts

@@ -323,7 +323,8 @@ export class DateGeneratorBaseStrategy {
 
       const duration = source.endDate.getTime() - source.startDate.getTime();
       const startDate = this.timeZoneCalculator.createDate(source.startDate, { path: 'toGrid' });
-      const endDate = dateUtilsTs.addOffsets(startDate, [duration]);
+      const endDate = this.timeZoneCalculator.createDate(source.endDate, { path: 'toGrid' });
+      // const endDate = dateUtilsTs.addOffsets(startDate, [duration]);
 
       return {
         startDate,

packages/devextreme/js/__internal/scheduler/appointments/resizing/m_core.ts

@@ -35,15 +35,15 @@ const getDateRangeHorizontal = (options) => {
   const appointmentCellsAmount = Math.round(relativeAppointmentRect.width / cellWidth);
   const appointmentLastCellIndex = appointmentFirstCell.index + (appointmentCellsAmount - 1);
 
-  const { sourceAppointment } = appointmentSettings.info;
+  const { appointment } = appointmentSettings.info;
   const { allDay } = appointmentSettings.info.appointment;
 
   if (handles.left) {
     return {
       startDate: appointmentFirstCell.startDate,
-      endDate: appointmentFirstCell.startDate > sourceAppointment.endDate
+      endDate: appointmentFirstCell.startDate > appointment.endDate
         ? appointmentFirstCell.startDate
-        : sourceAppointment.endDate,
+        : appointment.endDate,
     };
   }
 
@@ -60,9 +60,9 @@ const getDateRangeHorizontal = (options) => {
     : appointmentLastCell.startDate;
 
   return {
-    startDate: endDate < sourceAppointment.startDate
+    startDate: endDate < appointment.startDate
       ? endDate
-      : sourceAppointment.startDate,
+      : appointment.startDate,
     endDate,
   };
 };

packages/devextreme/js/__internal/scheduler/r1/timezone_calculator/calculator.ts

@@ -2,6 +2,7 @@ import dateUtils from '@js/core/utils/date';
 import { isDefined } from '@js/core/utils/type';
 import { dateUtilsTs } from '@ts/core/utils/date';
 
+import timeZoneUtils from '../../m_utils_time_zone';
 import { PathTimeZoneConversion } from './const';
 import type { DateType, TimeZoneCalculatorOptions, TimeZoneOffsetsType } from './types';
 
@@ -63,13 +64,22 @@ export class TimeZoneCalculator {
     const direction = isBack
       ? -1
       : 1;
-
     const resultDate = new Date(date);
-    return dateUtilsTs.addOffsets(resultDate, [
+    let convertedDateByOffsets = dateUtilsTs.addOffsets(resultDate, [
       direction * (toMs('hour') * targetOffset),
       -direction * (toMs('hour') * clientOffset),
     ]);
 
+    if (isBack) {
+      const DSTOffset = timeZoneUtils.getDaylightOffsetInMs(resultDate, convertedDateByOffsets);
+
+      if (DSTOffset !== 0) {
+        convertedDateByOffsets = dateUtilsTs.addOffsets(convertedDateByOffsets, [DSTOffset]);
+      }
+    }
+
+    return convertedDateByOffsets;
+
     // V1
     // NOTE: Previous date calculation engine.
     // Engine was changed after fix T1078292.