Edit the time string format.
        
        See https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior
        for all format string options.
        
        **中文文档**
        
        将datetime string从一种格式转换成另一种格式。

def reformat(self, dtstring, before, after):
        """Edit the time string format.
        
        See https://docs.python.org/2/library/datetime.html#strftime-and-strptime-behavior
        for all format string options.
        
        **中文文档**
        
        将datetime string从一种格式转换成另一种格式。
        """
        a_datetime = datetime.strptime(dtstring, before)
        return datetime.strftime(a_datetime, after)