public DateTimeStamp minus(double offsetInDecimalSeconds) {
        return add(-offsetInDecimalSeconds);
    }
@Test
    void testMinus() {
        DateTimeStamp a = new DateTimeStamp(.586);
        DateTimeStamp b = new DateTimeStamp(.587);
        double diff = a.minus(b);
        assertEquals(.586 - .587, diff, .001);

        a = new DateTimeStamp("2018-04-04T09:10:00.586-0100");
        b = new DateTimeStamp("2018-04-04T09:10:00.587-0100");
        diff = a.minus(b);
        double expected = -0.001d;
        assertEquals(expected, diff, 0.001);

        a = new DateTimeStamp("2018-04-04T09:10:00.586-0100", .18);
        b = new DateTimeStamp("2018-04-04T09:10:00.587-0100", .19);
        diff = a.minus(b);
        assertEquals(.18 - .19, diff, .001);

    }