public boolean after(DateTimeStamp other) {
        return compareTo(other) > 0;
    }
@Test
    void malformedDateTimeStampThrowsException() {
        final DateTimeStamp onlyDate = new DateTimeStamp("2021-09-01T11:12:13.111-0100");
        final DateTimeStamp onlyTime = new DateTimeStamp(1.0D);
        assertThrows(IllegalStateException.class, () -> onlyDate.after(onlyTime));
    }