DemoController.php

<?php
// ...
use Symfony\Component\Form\Extension\Core\DataTransformer\DateTimeToStringTransformer;
use Acme\DemoBundle\Entity\Post;

    public function indexAction()
    {
        $post = new Post;
        $builder = $this->createFormBuilder($post);
        $form = $builder
            ->add('title', 'text')
            ->add(
                $builder->create('date', 'hidden')
                    ->addViewTransformer(new DateTimeToStringTransformer)
            )
            ->add('submit', 'submit')
            ->getForm();

        $form->handleRequest($this->getRequest());

        if ($form->isValid()) {
            // ...
            return new Response($post->title.' on '.$post->date->format('d-m-Y'));
        }

        return array('form' => $form->createView());
    }

Post.php

<?php

namespace Acme\DemoBundle\Entity;

use Symfony\Component\Validator\Constraints as Assert;

class Post
{
    /**
     * @var \DateTime
     *
     * @Assert\Date()
     */
    public $date;

    public $title;

    public function __construct()
    {
        $this->date = new \DateTime();
    }
}