main.dart

void main() {
  
  String arrived = "07:45";
  String lunched = "11:40";
  String returned = "12:09";
  String leave_time = getLeaveTime(arrived, lunched, returned);
  
  print("-------\n->$arrived\n->$lunched\n->$returned\n->$leave_time");
}

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

String getLeaveTime(String arrived, String lunched, String returned){
  int daily_needed_minutes = 480; // 8 hours;
  var min_arrival_date = parseDate("07:30");
  var arrived_date = parseDate(arrived);
  var returned_date = parseDate(returned);
  var lunched_date = parseDate(lunched);
  
  if (arrived_date.isBefore(min_arrival_date)){ // prevents arrival before 07:30
    arrived = "07:30";
  }
  
  var lunch_time = returned_date.difference(lunched_date);
  if (lunch_time.inMinutes < 30){ // prevents lunch time less than half hour
    returned_date = lunched_date.add(Duration(minutes: 30));
  }

  var morning_minutes = lunched_date.difference(arrived_date).inMinutes; // minutes worked in the morning 
  var needed_minutes = daily_needed_minutes - morning_minutes; 
  var leave = returned_date.add(Duration(minutes: needed_minutes));
  
  String leave_time = "${leave.hour.toString().padLeft(2,'0')}:${leave.minute.toString().padLeft(2,'0')}";
  return leave_time;
}