Test Booking Object with PHPUnit

<?php
 namespace BookingNamespace;

     // Override DateTime in current namespace for testing
    require_once "DateTime.class.php";
    require_once "../MyBookingObject.class.php";

    class MyBookingObjectTest extends \PHPUnit_Framework_TestCase
    {

        private $booking_object;

        /**
         * Create test subject before test
         */
        protected function setUp()
        {
            parent::setUp();
            DateTime::set_now("2014-04-02 16:15");
            $this->booking_object = new MyBookingObject();
        }

        /**
         * Reset custom time after test
         */
        protected function tearDown()
        {
            DateTime::reset();
        }

        /*
         * Test cases
         */
        public function testBookAndConfirm()
        {
            $token = $this->booking_object->book();
            $this->assertTrue($this->booking_object->confirm($token));
        }
        public function testBookAndConfirmThreeMinutesLater()
        {
            $token = $this->booking_object->book();
            DateTime::modify_now("+3 minutes");
            $this->assertTrue($this->booking_object->confirm($token));
        }
        public function testBookAndConfirmSixMinutesLater()
        {
            $token = $this->booking_object->book();
            DateTime::modify_now("+6 minutes");
            $this->assertFalse($this->booking_object->confirm($token));
        }
    }

?>