def get_start_time_str(self):
        """
        :return:
            |attr_start_datetime| as |str| formatted with
            |attr_start_time_format|.
            Return |NaT| if the invalid value or the invalid format.
        :rtype: str

        :Sample Code:
            .. code:: python

                from datetimerange import DateTimeRange
                time_range = DateTimeRange("2015-03-22T10:00:00+0900", "2015-03-22T10:10:00+0900")
                print(time_range.get_start_time_str())
                time_range.start_time_format = "%Y/%m/%d %H:%M:%S"
                print(time_range.get_start_time_str())
        :Output:
            .. parsed-literal::

                2015-03-22T10:00:00+0900
                2015/03/22 10:00:00
        """

        try:
            return self.start_datetime.strftime(self.start_time_format)
        except AttributeError:
            return self.NOT_A_TIME_STR
:return:
            |attr_start_datetime| as |str| formatted with
            |attr_start_time_format|.
            Return |NaT| if the invalid value or the invalid format.
        :rtype: str

        :Sample Code:
            .. code:: python

                from datetimerange import DateTimeRange
                time_range = DateTimeRange("2015-03-22T10:00:00+0900", "2015-03-22T10:10:00+0900")
                print(time_range.get_start_time_str())
                time_range.start_time_format = "%Y/%m/%d %H:%M:%S"
                print(time_range.get_start_time_str())
        :Output:
            .. parsed-literal::

                2015-03-22T10:00:00+0900
                2015/03/22 10:00:00