Symfony/Doctrine: DateTime as primary key

I am trying to make an Entity using a date as a primary key. The problem is that Symfony can't convert the DateTime I'm using into a string to introduce it in the IdentityMap. I get the following error during the persist of the entity:

Catchable Fatal Error: Object of class DateTime could not be converted to string in..

I'm using this code in the entity:

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

The error appears in the entity repository:

$em = $this->getEntityManager();
$currentData = new CurrentData();
...
$currentData->setDate(new \DateTime($dateStr));
...
$em->persist($currentData);
$em->flush();

How can I solve this problem? Thank you.