title: Django F expression on datetime objects
body: <p>My model is:</p>
  <pre><code>class Test():
     date1 = models.DateTimeField()
     date2 = models.DateTimeField()
  </code></pre>
  <p>I can find out objects whose <code>date2</code> is greater than <code>date1</code>, using the following query:</p>
  <pre><code>Test.objects.filter(date2__gt=F('date1'))
  </code></pre>
  <p>I would like to find all the objects whose <code>date2</code> is greater than <code>date1</code> by one year.<br>
  How can I find out objects based on difference between <code>date1</code> and <code>date2</code>?</p>