code: func (ts Timestamp) String() string {
  	if ts.IsZero() {
  		return ""
  	}
  	return fmt.Sprintf("%d-%02d-%02dT%02d:%02d:%02d.%03dZ",
  		ts.Year(), ts.Month(), ts.Day(),
  		ts.Hour(), ts.Minute(), ts.Second(), ts.Nanosecond()/1000000)
  }
docs: //
  // String - Show the Timestamp in RFC3339 format, UTC, millisecond resolution
  //