docs: def add_seconds(self, datetimestr, n):
          """Returns a time that n seconds after a time.
  
          :param datetimestr: a datetime object or a datetime str
          :param n: number of seconds, value can be negative
  
          **中文文档**
  
          返回给定日期N秒之后的时间。
          """
          a_datetime = self.parse_datetime(datetimestr)
          return a_datetime + timedelta(seconds=n)