Get correct ISO DateTime String with offset in Flutter: https://dartpad.dev/84d855e41c0134a34ff8b2cf034ad249

date_utils.dart

import 'package:intl/intl.dart';
import 'package:meta/meta.dart';

void main() {
  print(DateUtils.formatISOTime(DateTime.now()));

  print(DateUtils.getCurrentISOTimeString());
}

class DateUtils {
 
 ///Converts DateTime to ISO format 
 ///Output: 2020-09-16T20:41:09.331+05:30
  static String formatISOTime(DateTime date) {
    var duration = date.timeZoneOffset;
    if (duration.isNegative)
      return (date.toIso8601String() +
          "-${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}");
    else
      return (date.toIso8601String() +
          "+${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}");
  }

 ///get ISO time String from DateTime
 ///Output: 2020-09-16T20:42:38.629+05:30
  static String getCurrentISOTimeString({DateTime dateTime}) {
    var date = dateTime ?? DateTime.now();
    //Time zone may be null in dateTime hence get timezone by  datetime
    var duration = DateTime.now().timeZoneOffset;
    if (duration.isNegative)
      //TODO: convert duration to abs value instead of below params
      return (date.toIso8601String() +
          "-${duration.inHours.abs().toString().padLeft(2, '0')}:${(duration.inMinutes.abs() - (duration.inHours.abs() * 60)).toString().padLeft(2, '0')}");
    else
      return (date.toIso8601String() +
          "+${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}");
  }
}

@loonix, the code will not have a double minus if it's using the absolute value of the duration (.abs()), but your suggestion would also work.