public function testAdd() { $d1 = new Duration('PT1S'); $d2 = new Duration('PT1S'); $d1->add($d2); $this->assertEquals('PT2S', $d1->__toString()); $d1 = new Duration('PT23H59M59S'); $d2 = new Duration('PT10S'); $d1->add($d2); $this->assertEquals('P1DT9S', $d1->__toString()); }