docs: def rnd_datetime(self, start=datetime(1970, 1, 1), end=datetime.now()):
          """Generate a random datetime between ``start`` to ``end``.
  
          :param start: Left bound
          :type start: string or datetime.datetime, (default datetime(1970, 1, 1))
          :param end: Right bound
          :type end: string or datetime.datetime, (default datetime.now())
          :return: a datetime.datetime object
  
          **中文文档**
  
          随机生成一个位于 ``start`` 和 ``end`` 之间的时间。
          """
          if isinstance(start, string_types):
              start = self.str2datetime(start)
          if isinstance(end, str):
              end = self.str2datetime(end)
          if start > end:
              raise ValueError("start time has to be earlier than end time")
          return self.from_utctimestamp(
              random.randint(
                  int(self.to_utctimestamp(start)),
                  int(self.to_utctimestamp(end)),
              )
          )