code: func (t Timestamp) MarshalJSON() ([]byte, error) {
  	if t.Location() != time.UTC {
  		return nil, errors.New("Timestamp.MarshalJSON: time zone must be UTC")
  	}
  	if y := t.Year(); y < 0 || 9999 < y {
  		// RFC 3339 is clear that years are 4 digits exactly.
  		// See golang.org/issue/4556#issuecomment-66073163 for more discussion.
  		return nil, errors.New("Timestamp.MarshalJSON: year outside of range [0,9999]")
  	}
  	b := make([]byte, 0, len(timeLayout)+2)
  	b = append(b, '"')
  	b = t.AppendFormat(b, timeLayout)
  	b = append(b, '"')
  	return b, nil
  }
docs: // MarshalJSON implements the json.Marshaler interface.
  // The time is a quoted string in Gerrit's timestamp format.
  // An error is returned if t.Time time zone is not UTC.