cal.set(Calendar.YEAR, 1);
assertEquals("0001-01-01T00:00:00.000Z", fmt.format(cal.getTime()));
// according to the spec, the year before is year 0000
cal.add(Calendar.YEAR, -1);
assertEquals("0000-01-01T00:00:00.000Z", fmt.format(cal.getTime()));
// and now where negative territory
cal.add(Calendar.YEAR, -1);
assertEquals("-0001-01-01T00:00:00.000Z", fmt.format(cal.getTime()));