title: Symfony2 - No default option is configured for constraint
body: <p>I was following tutorial book for Symfony2.
  I was doing project with forms.</p>
  
  <p>Chapter 12: Forms | 150</p>
  
  <p>My code:</p>
  
  <pre><code>    public function newAction(Request $request)
      {
     $task = new Task();
     $task-&gt;setTask('Find EiM group');
     $task-&gt;setDueDate(new DateTime('tomorrow'));
  
     $form = $this-&gt;createFormBuilder($task)
        -&gt;add('task', 'text')
        -&gt;add('dueDate', 'date')
        -&gt;add('save', 'submit')
        -&gt;getForm();
  
     return $this-&gt;render('AcmeTaskBundle:Default:new.html.twig', array(
         'form' =&gt; $form-&gt;createView()));
      }
  </code></pre>
  
  <p>And I'm getting error from the topic.</p>
  
  <pre><code>No default option is configured for constraint Symfony\Component\Validator\Constraints\DateTime
  </code></pre>
  
  <p>What it can be?
  How to fix it?
  I followed example in book strictly.</p>