flutter_local_notification chronometerCountDown going upward only

I am actually showing a notification that indicates the remaining time of the Booking of the Vehicle parked at the parking lot.

eg. From Booking_Time = 2022-10-10 12:25 To Booking_Time = 2022-10-10 18:25

so the problem is, suppose the time remaining is 06:25:00. In the notification it starts properly but no matter what the value for chronometerCountDown = true/false the timer goes upward only. if chronometerCountDown = false it shows 06:25:00, 06:25:01, 06:25:02, 06:25:03...this is as expected if chronometerCountDown = true it shows -06:25:00, -06:25:01, -06:25:02, -06:25:03...going upward only

Below is my AndroidNOtificationDetails ::

DateTime whenTimer =
    currentTime.subtract(durationElapsed);
AndroidNotificationDetails and = AndroidNotificationDetails(
  'sample_vehicle',
  'Vehicle Parking Time Remaining',
  channelDescription:
      'Notify the user that vehicle\'s time of booking...',
  importance: Importance.max,
  priority: Priority.max,
  channelShowBadge: false,
  ticker: 'sample_vehicle',
  color: Colors.blue,
  onlyAlertOnce: true,
  when: whenTimer.millisecondsSinceEpoch,
  timeoutAfter: whenTimer.difference(DateTime.now()).inMilliseconds,
  usesChronometer: true,
  chronometerCountDown: true, 
  visibility: NotificationVisibility.public,
  ongoing: true,
);

I hope I have mentioned my problem properly.

NOTE: I have checked the example and it shows same behaviour

when: DateTime.now().millisecondsSinceEpoch - 120 * 1000, usesChronometer: true, chronometerCountDown: true,

In short chronometerCountDown: true only adds the "-" sign nothing else.

Here I got the answer to my question on Github forum of the flutter_local_notification only.

If we use chronometerCountDown: false the value of when: DateTime.now().millisecondsSinceEpoch - 120 * 1000, shows we have subtracted 2 mins from the current epoch time and so it will count up to fill the gap.

Since I was counting down, I needed to add to the when time, not subtract means, when: DateTime.now().millisecondsSinceEpoch + 120 * 1000, which shows that we have added 2 mins and now it has to count down to fill the gap.

currentTime.subtract(durationElapsed); //Here countUp will work currentTime.add(durationElapsed); //Here countDown will work

Here is the full example code :

DateTime currentTime = DateTime.now();
            DateTime whenTimer = currentTime.add(durationElapsed);
        AndroidNotificationDetails and = AndroidNotificationDetails(
          'sample_vehicle',
          'Vehicle Parking',
          channelDescription:
              'Notify the user that vehicle\'s time of booking ...',
          importance: Importance.max,
          priority: Priority.max,
          channelShowBadge: true,
          ticker: 'sample_vehicle',
          color: Colors.blue,
          onlyAlertOnce: true,
          when: whenTimer.millisecondsSinceEpoch,
          timeoutAfter: whenTimer.difference(currentTime).inMilliseconds,
          usesChronometer: true,
          chronometerCountDown: true,
          visibility: NotificationVisibility.public,
          ongoing: true,
        );