def get_start_time_str(self):
try:<EOL><INDENT>return self.start_datetime.strftime(self.start_time_format)<EOL><DEDENT>except AttributeError:<EOL><INDENT>return self.NOT_A_TIME_STR<EOL><DEDENT>
: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