public void testFactory_between_RInstant() {
        // test using Days
        DateTime start = new DateTime(2006, 6, 9, 12, 0, 0, 0, PARIS);
        DateTime end1 = new DateTime(2006, 6, 12, 12, 0, 0, 0, PARIS);
        DateTime end2 = new DateTime(2006, 6, 15, 18, 0, 0, 0, PARIS);
        
        assertEquals(3, Single.between(start, end1, DurationFieldType.days()));
        assertEquals(0, Single.between(start, start, DurationFieldType.days()));
        assertEquals(0, Single.between(end1, end1, DurationFieldType.days()));
        assertEquals(-3, Single.between(end1, start, DurationFieldType.days()));
        assertEquals(6, Single.between(start, end2, DurationFieldType.days()));
        try {
            Single.between(start, (ReadableInstant) null, DurationFieldType.days());
            fail();
        } catch (IllegalArgumentException ex) {
            // expected
        }
        try {
            Single.between((ReadableInstant) null, end1, DurationFieldType.days());
            fail();
        } catch (IllegalArgumentException ex) {
            // expected
        }
        try {
            Single.between((ReadableInstant) null, (ReadableInstant) null, DurationFieldType.days());
            fail();
        } catch (IllegalArgumentException ex) {
            // expected
        }
    }