protected long calculatePaneStart(long timeMillis) {
        return timeMillis - timeMillis % paneIntervalInMs;
    }
@Test
    void testCalculatePaneStart() {
        long time = System.currentTimeMillis();
        assertTrue(window.calculatePaneStart(time) <= time);
        assertTrue(time < window.calculatePaneStart(time) + window.getPaneIntervalInMs());
    }