code: def date_time_string(timestamp=None):
      
      global _last_date_time_string
      _last_timestamp, _last_str = _last_date_time_string
      if timestamp is None:
          timestamp = time.time()
      _curr_timestamp = int(timestamp)
      if _curr_timestamp == _last_timestamp:
          return _last_str
      else:
          year, month, day, hh, mm, ss, wd, y, z = time.gmtime(timestamp)
          s = b"%s, %02d %3s %4d %02d:%02d:%02d GMT" % (
                  weekdayname[wd],
                  day, monthname[month], year,
                  hh, mm, ss)
          _last_date_time_string = (_curr_timestamp, s)
          return s
docs: Return the current date and time formatted for a message header.