public void testMinus_Days() {
        Days test2 = Days.days(2);
        Days test3 = Days.days(3);
        Days result = test2.minus(test3);
        assertEquals(2, test2.getDays());
        assertEquals(3, test3.getDays());
        assertEquals(-1, result.getDays());
        
        assertEquals(1, Days.ONE.minus(Days.ZERO).getDays());
        assertEquals(1, Days.ONE.minus((Days) null).getDays());
        
        try {
            Days.MIN_VALUE.minus(Days.ONE);
            fail();
        } catch (ArithmeticException ex) {
            // expected
        }
    }