@Test public void relativeTimestamp_incrementsAccordingToNanoTick() {

  mockStatic(System.class);
  TickClock clock = new TickClock(1000L /* 1ms */, 0L /* 0ns */);
  when(System.nanoTime()).thenReturn(1000L); // 1 microsecond = 1000 nanoseconds
  assertThat(clock.currentTimeMicroseconds()).isEqualTo(1001L); // 1ms + 1us
}