time_calc.dart

void main() {
  
  int daily_needed_minutes = 480; // 8 hours;
  var min_arrival_date = getParsedDate("07:30");
  
  String arrived = "07:31";
  String lunched = "11:30";
  String returned = "12:34";
  
  var arrived_date = DateTime.parse("2019-31-10 $arrived:00Z");
  if (arrived_date.isBefore(min_arrival_date)){
    arrived = "07:30";
  }
  
  var morning = DateTime.parse("2019-31-10 $lunched:00Z").difference(DateTime.parse("2019-31-10 $arrived:00Z"));  
  int morning_minutes = morning.inMinutes;
  
  print(morning_minutes);
  
 
  var needed_minutes = daily_needed_minutes - morning_minutes;
  print(needed_minutes);
  
  var leave = DateTime.parse("2019-31-10 $returned:00Z").add(Duration(minutes: needed_minutes));
  
  String leave_at = "${leave.hour.toString().padLeft(2,'0')}:${leave.minute.toString().padLeft(2,'0')}";
	print(leave_at);
  print(getParsedDate("07:30"));
  print(DateTime.now());
}

DateTime getParsedDate(time){
  var today = DateTime.now().toString();
  String parsed_date = today.substring(0, 11) + time + ":00.000Z";
  return DateTime.parse(parsed_date);
}