code: func fastTimeMarshalJSON(t time.Time) (string, error) {
  	if y := t.Year(); y < 0 || y >= 10000 {
  		// RFC 3339 is clear that years are 4 digits exactly.
  		// See golang.org/issue/4556#c15 for more discussion.
  		return "", errors.New("time.MarshalJSON: year outside of range [0,9999]")
  	}
  	return t.Format(jsonFormat), nil
  }
docs: // fastTimeMarshalJSON avoids one of the extra allocations that
  // time.MarshalJSON is making.