Reply for /r/dartlang - https://www.reddit.com/r/dartlang/comments/bpfszt/return_length_of_time_between_two_strings_of_hhmm/

timediff.dart

Duration toDuration(String duration) {
  var tokens = duration.split(":").map(int.parse).toList();
  
  if (tokens.length == 3)
    return Duration(hours: tokens[0], minutes: tokens[1], seconds: tokens[2]);
  else
    return Duration(hours: tokens[0], minutes: tokens[1]);
}

Duration timediff(String startTime, String endTime) {
  var startTimeDuration = toDuration(startTime);
  var endTimeDuration = toDuration(endTime);
  
  if (endTimeDuration.compareTo(startTimeDuration) >= 0)
    return endTimeDuration - startTimeDuration;
  else
    return (endTimeDuration + Duration(hours: 24)) - startTimeDuration;
}

main() {
  print(timediff("15:12", "16:21"));
  print(timediff("13:55", "23:12"));
  print(timediff("21:15", "13:09"));
  print(timediff("16:55", "00:11"));
  print(timediff("21:16", "14:41"));
  print(timediff("12:55", "15:02"));
}