code: def get_start_time_str(self):
          
  
          try:
              return self.start_datetime.strftime(self.start_time_format)
          except AttributeError:
              return self.NOT_A_TIME_STR
docs: :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