title: Multiple fields for one attribute
body: <p>Say I have three fields for a user to enter a date: day, month, and year. (Likely not the best way, but I'm using it for this example). If I try to <code>$form-&gt;bind()</code> the user's request, Symfony will try and look for a <code>-&gt;setMonth()</code> method on my model, which doesn't exist.</p>
  
  <p>How do I tell Symfony that I want to run the input through <code>strtotime()</code> (or any other any other arbitrary code or method) before, and then map the new output to <code>setDate()</code>?</p>