How to select a week(from Monday to Sunday) in the Flutter Date Range Picker (SfDateRangePicker)

I try to implement a weekly selector by following your sample code - https://www.syncfusion.com/kb/11412/how-to-select-a-week-in-the-flutter-date-range-picker-sfdaterangepicker

The problem I encounter is that the "args.value" from DateRangePickerSelectionChangedArgs returns a date range from Sunday to Saturday. What I want is for the DateRangePickerSelectionChangedArgs to return a date range from Monday to Sunday. I want the weekly selector to select the whole week from Monday to Sunday not from Sunday to Saturday.

I try the codes below. I tried adding one to the start date and to the end date so that Sunday becomes Monday and Saturday becomes Sunday, but the code didn't work when I did that.

  void selectionChanged(DateRangePickerSelectionChangedArgs args) {
      isSameDate(date1, date2) {
        if (date2 == date1) {
          return true;
        }
        if (date1 == null || date2 == null) {
          return false;
        }
        return date1.month == date2.month && date1.year == date2.year && date1.day == date2.day;
      }
      int firstDayOfWeek = DateTime.sunday % 7;
      int endDayOfWeek = (firstDayOfWeek - 1) % 7;
      endDayOfWeek = endDayOfWeek < 0 ? 7 + endDayOfWeek : endDayOfWeek;
      PickerDateRange ranges = args.value;

      DateTime date1 = ranges.startDate!;
      DateTime date2 = (ranges.endDate ?? ranges.startDate)!;

      if (date1.isAfter(date2)) {
        var date = date1;
        date1 = date2;
        date2 = date;
      }
      int day1 = date1.weekday % 7;
      int day2 = date2.weekday % 7;

      DateTime dat1 = date1.add(Duration(days: (firstDayOfWeek - day1) + 1));
      DateTime dat2 = date2.add(Duration(days: (endDayOfWeek - day2) + 1));

      if (!isSameDate(dat1, date1) || !isSameDate(dat2, date2)) {
        datePickerController.selectedRange = PickerDateRange(dat1, dat2);
      }
  }