public static @Nullable Instant windmillToHarnessWatermark(long watermarkUs) {
    if (watermarkUs == Long.MIN_VALUE) {
      // Not yet known.
      return null;
    } else {
      return windmillToHarnessTimestamp(watermarkUs);
    }
  }
@Test
  public void testWindmillToHarnessWatermark() {
    assertEquals(null, windmillToHarnessWatermark(Long.MIN_VALUE));
    assertEquals(BoundedWindow.TIMESTAMP_MAX_VALUE, windmillToHarnessWatermark(Long.MAX_VALUE));
    assertEquals(
        BoundedWindow.TIMESTAMP_MAX_VALUE, windmillToHarnessWatermark(Long.MAX_VALUE - 17));
    assertEquals(new Instant(16), windmillToHarnessWatermark(16999));
    assertEquals(new Instant(17), windmillToHarnessWatermark(17120));
    assertEquals(new Instant(17), windmillToHarnessWatermark(17000));
    assertEquals(new Instant(-17), windmillToHarnessWatermark(-16987));
    assertEquals(new Instant(-17), windmillToHarnessWatermark(-17000));
    assertEquals(new Instant(-18), windmillToHarnessTimestamp(-17001));
  }