public static void rollMockClock(Duration delta) {
        if (clock.equals(Clock.systemUTC()))
            throw new IllegalStateException("You need to use setMockClock() first.");
        setMockClock(clock.instant().plus(delta));
    }
@Test(expected = IllegalStateException.class)
    public void rollMockClock_uninitialized() {
        TimeUtils.rollMockClock(Duration.ofMinutes(1));
    }