@Test
 public void durationBetween4() {
   Timestamp reference = Timestamp.of(10, 500000000);
   assertThat(
       reference.durationBetween(reference.minus(TimeDuration.ofMillis(600))),
       equalTo(TimeDuration.ofMillis(600)));
 }
 @Test
 public void durationBetween2() {
   Timestamp reference = Timestamp.now();
   assertThat(
       reference.durationBetween(reference.minus(TimeDuration.ofNanos(10))),
       equalTo(TimeDuration.ofNanos(10)));
 }