From http://www.jamesmandrews.com/2012/10/31/symfony-2-entity-timestamps-with-doctrine/

Symfony2 updatetimestamps

namespace Acme\ProjectBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity
 * @ORM\HasLifecycleCallbacks
 * @ORM\Table(name="projects")
 */

class Projects
{
    /**
    * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ORM\Column(type="string")
     */
    protected $name;

    /**
     * @ORM\Column(type="datetime")
     */
    protected $created_at;

    /**
     * @ORM\Column(type="datetime")
     */
    protected $modified_at;

    /**
     * Now we tell doctrine that before we persist or update we call the updatedTimestamps() function.
     *
     * @ORM\PrePersist
     * @ORM\PreUpdate
     */
    public function updatedTimestamps()
    {
        $this->setModifiedAt(new \DateTime(date('Y-m-d H:i:s')));

        if($this->getCreatedAt() == null)
        {
            $this->setCreatedAt(new \DateTime(date('Y-m-d H:i:s')));
        }
    }
}

NOTE: @ORM\HasLifecycleCallbacks must be