code: func (v *Validator) validTimestamp(ts string) bool {
  	t, err := stringToTime(ts)
  	if err != nil {
  		return false
  	}
  	diff := time.Now().Add(ValidityWindow / 2).Sub(t)
  	return diff < ValidityWindow && diff > 0
  }
docs: // validTimestamp validates if the MessageBird-Request-Timestamp is a valid
  // date and if the request is older than the validator Period.