title: Validating that a date is greater or equal than another date, without entity
body: <p>I built a form not related to any entity.</p>
  
  <pre><code>class CalculatorType extends AbstractType
  {
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder
          -&gt;add('debutActivite', 'date', array(
              'label' =&gt; 'Quand avez-vous commencé votre activité ?',
              'widget' =&gt; 'single_text',
              'format' =&gt; 'dd/MM/yyyy',
              'label_attr' =&gt; array(
                  'class' =&gt; 'control-label',
              ),
              'attr' =&gt; array(
                  'class' =&gt; 'form-control createDatepicker',
                  'placeholder' =&gt; 'dd/mm/aaaa',
              ),
              'required' =&gt; true,
          ))
          -&gt;add('demandeAccreAccepteeDate', 'date', array(
              'label' =&gt; 'A quelle date a-t-elle été acceptée ?',
              'widget' =&gt; 'single_text',
              'format' =&gt; 'dd/MM/yyyy',
              'label_attr' =&gt; array(
                  'class' =&gt; 'control-label',
              ),
              'attr' =&gt; array(
                  'class' =&gt; 'form-control createDatepicker',
                  'placeholder' =&gt; 'dd/mm/aaaa',
              ),
          ))
      }
  }
  </code></pre>
  
  <p>And in my controller, I check if form is valid :</p>
  
  <pre><code>public function indexAction(Request $request)
  {
      $form = $this-&gt;createForm(new CalculatorType(), array());
  
      $form-&gt;handleRequest($request);
  
      if ($form-&gt;isValid()) {
          $data = $form-&gt;getData();
      }
  }
  </code></pre>
  
  <p>The "demandeAccreAccepteeDate" field is not required. But if it's not empty, i would like to validate that it is greater or equal to "debutActivite" field.</p>
  
  <p>How can I do such a thing, knowing that I have no Calculator entity ?</p>