code: def reformat(self, dtstring, before, after):
          
          a_datetime = datetime.strptime(dtstring, before)
          return datetime.strftime(a_datetime, after)
docs: 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。