normalizeDateFromController function

Implementation

DateTime? normalizeDateFromController(
  DateTime? date,
  DateTime? controllerTime,
  DateTime currentTime,
) {
  if (date == null || controllerTime == null) {
    return date;
  }

  // Backend currently stores dates based on the controller's time, but then
  // sends back the dates using the server's timezone.
  //
  // For example, an event that happens 5PM ET from the controller is stored
  // in the backend as 5PM (with no timezone value). Then, the backend service
  // sends the time as 5PM CT. See below table.
  //
  //     5PM EST               5PM         5PM CST / 6PM EST      3 PM PST
  //   Event time        Time stored in      Time sent by      Time displayed
  // from controller        database           backend            to user
  //
  // The frontend can correctly convert the time to the user's local
  // timezone, but we still need to correct for the difference between the
  // controller's time and the server's time.

  // First, get the current server time. Per Ashok, backend servers use
  // US / Central Time.
  final serverTzOffset = Duration(
    milliseconds: tz
        .getLocation('US/Central')
        .timeZone(currentTime.millisecondsSinceEpoch)
        .offset,
  );
  final tzDifference = serverTzOffset - currentTime.timeZoneOffset;
  final serverTime = currentTime.add(tzDifference);

  // Then get the difference between the controller time and the server.
  var difference = serverTime.difference(controllerTime);

  final inMinutes = difference.inMinutes;
  final inHours = (inMinutes / 60.0).round();

  final minutesRemainder = (inMinutes - (inHours * 60)).abs();
  if (minutesRemainder <= 5) {
    // If the difference is close to a multiple of an hour, round to the
    // nearest hour. This means the controller time is up-to-date, but
    // potentially in a different timezone.
    difference = Duration(hours: inHours);
  }

  // Add the difference to the event date
  return date.add(difference);
}