title: Modify date in fluid's f:format.date - using inline notation
body: <p>With the fluid <code>f:format.date</code> viewhelper, I can set an offset on a date:</p>
  
  <pre><code>&lt;f:format.date format="%d.%m.%Y"&gt;{item.Date}-3 days&lt;/f:format.date&gt;
  </code></pre>
  
  <p>Unfortunately, it's not that easy with the inline notation (which I need for a condition). I've tried the following with :</p>
  
  <pre><code>{f:format.date(date: item.Date-3 days, format: '%d.%m.%Y')}
  {f:format.date(date: item.Date'-3 days', format: '%d.%m.%Y')}
  {f:format.date(date: 'item.Date-3 days', format: '%d.%m.%Y')}
  </code></pre>
  
  <p>None of which returns a modified date.
  What is the correct syntax?</p>