public function testHowToAddRemoveADurationFromItInDays() { $jan1 = new Date(2013, 1, 1); $dec31 = $jan1->minus(new Duration(1, TimeUnit::day())); $jan3 = $jan1->plus(new Duration(2, TimeUnit::day())); $this->assertEquals(new Date(2012, 12, 31), $dec31); $this->assertEquals(new Date(2013, 1, 3), $jan3); }