title: Django filter objects F('duration') from now
body: <p>I have a model:</p>
  
  <pre><code>class Examle(models.Model):
    date = models.DateTime(auto_add_now=True)
    duration = models.IntegerField(default=90) # days
  </code></pre>
  
  <p>And I want to filter it like so:</p>
  
  <pre><code>Example.objects.filter(date_lt=datetime.today() - timedelta(days=F('duration')))
  </code></pre>
  
  <p>Of course <code>timedelta</code> won't take F-object as valid parameter, but hope you got what I mean.</p>