symfony2 Expression validation

Conference.php

use Symfony\Component\Validator\Constraints as Assert;


class Conference
{
    /**
     * @var \DateTime
     *
     * @Assert\Expression(
     *     "this.startDate <= this.endDate",
     *     message="Start date should be less or equal to end date!"
     * )
     */
    protected $startDate;

    /**
     * @var \DateTime
     *
     * @Assert\Expression(
     *     "this.endDate >= this.startDate",
     *     message="End date should be greater or equal to start date!"
     * )
     */
    protected $endDate;
}