public double elapsedSeconds() {
        return elapsed(TimeUnit.SECONDS);
    }
@Test
    void elapsedSeconds() {
        Supplier<Long> mockClock = mock(Supplier.class);
        when(mockClock.get()).thenReturn(SECONDS.toNanos(1), SECONDS.toNanos(3));
        Timer timer = new Timer(mockClock);
        assertThat(timer.elapsedSeconds()).isEqualTo(2.0d);
    }