Sets this message as flagged
        :param start_date: the start datetime of the followUp
        :param due_date: the due datetime of the followUp

def set_flagged(self, *, start_date=None, due_date=None):
        """ Sets this message as flagged
        :param start_date: the start datetime of the followUp
        :param due_date: the due datetime of the followUp
        """
        self.__status = Flag.Flagged
        start_date = start_date or dt.datetime.now()
        due_date = due_date or dt.datetime.now()
        if start_date.tzinfo is None:
            start_date = self.protocol.timezone.localize(start_date)
        if due_date.tzinfo is None:
            due_date = self.protocol.timezone.localize(due_date)
        self.__start = start_date
        self.__due_date = due_date
        self._track_changes()