// including timezone is ignored.
func dateDiff(timePart string, ts1, ts2 time.Time) (*Value, error) {
	if ts2.Before(ts1) {
		v, err := dateDiff(timePart, ts2, ts1)
		if err == nil {
			v.negate()
		}
		return v, err
	}

	duration := ts2.Sub(ts1)
	y1, m1, d1 := ts1.Date()
	y2, m2, d2 := ts2.Date()

	switch timePart {
	case timePartYear:
		dy := int64(y2 - y1)