func (t Time) Compare(u Time) int {
	var tc, uc int64
	if t.wall&u.wall&hasMonotonic != 0 {
		tc, uc = t.ext, u.ext
	} else {
		tc, uc = t.sec(), u.sec()
		if tc == uc {
			tc, uc = int64(t.nsec()), int64(u.nsec())
		}
	}
	switch {
	case tc < uc:
		return -1
	case tc > uc:
		return +1
	}
	return 0
}