code: def isotime(at=None, subsecond=False):
      
      if not at:
          at = utcnow()
      st = at.strftime(_ISO8601_TIME_FORMAT
                       if not subsecond
                       else _ISO8601_TIME_FORMAT_SUBSECOND)
      tz = at.tzinfo.tzname(None) if at.tzinfo else 'UTC'
      st += ('Z' if tz == 'UTC' else tz)
      return st
docs: Stringify time in ISO 8601 format.