Static mocks are sometimes necessary. You may have functions that perform date and/or time operations using the current time on the timeline at time of calling. Often these functions will reference the .now() static function which is present on most of the new Java time classes. Two examples I use frequently are LocalDateTime.now() and ZonedDateTime.now(). Due to the nature of these, writing tests which assert against known values is difficult. It would be much easier to write tests if we could guarantee the same time was returned for all our tests.

If you choose to mock .now() when testing your methods, you will run into issues when performing operations which add or subtract from the returned value, as an example. In order to mock calls to .now() while retaining the ability to perform operations on the returned value, we should instead mock the Clock:

class SandBoxTest {

    @Test
    void canMockDate_andPerformOperationsAgainst() {
        final Clock fixedClock = Clock.fixed(Instant.EPOCH, ZoneId.of("America/New_York"));
        try (MockedStatic<Clock> mockClock = Mockito.mockStatic(Clock.class)) {
            mockClock.when(Clock::systemDefaultZone).thenReturn(fixedClock);
            assertEquals(LocalDateTime.ofInstant(Instant.EPOCH, ZoneId.of("America/New_York")), LocalDateTime.now());
            // Can add and subtract
            assertEquals(LocalDateTime.ofInstant(Instant.EPOCH, ZoneId.of("America/New_York")).plusDays(1), LocalDateTime.now().plusDays(1));
        }
    }
}

Using Mockito, we can leverage the try-with-resources static mock functionality. First, outside your try block, create a Clock at a fixed time. Supply it the appropriate instant and timezone. This example uses Epoch. In the try-with-resources block, create a new MockedStatic<Clock> and configure it to return your fixed clock when .systemDefaultTimezone() is called.

And voilà! Classes which are backed by clocks can now return fixed times inside the try with resources block. This lets your write predictable tests for your date and time operations.