code: func ParseDuration(v string) (Duration, error) {
  	duration, err := time.ParseDuration(v)
  	if err != nil {
  		return 0, err
  	}
  	return Duration(duration), nil
  }
docs: // ParseDuration parses a clockflag Duration from a string. This is basically
  // a typed fall-through to time.ParseDuration.