Get start time as string (formatted with ``start_time_format``)
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: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:
    ::

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