code: @Test public void shouldNotExceedNumberNegative() throws Exception {
    Clock testingClock=new DefaultClockImpl();
    testingClock.setCurrentTime(parse("19700101-00:00:00"));
    DurationHelper dh=new DurationHelper("R2/PT10S/1970-01-01T00:00:50",testingClock);
    testingClock.setCurrentTime(parse("19700101-00:00:20"));
    assertEquals(parse("19700101-00:00:30"),dh.getDateAfter());
    testingClock.setCurrentTime(parse("19700101-00:00:35"));
    assertEquals(parse("19700101-00:00:35"),dh.getDateAfter());
  }
  