title: get average of difference of datetime fields in django
body: <p>In a django application, I have a model like following:</p>
  
  <pre><code>class Doe(models.Model):
      start = models.DateTimeField()
      end = models.DateTimeField()
  </code></pre>
  
  <p>I want to find the average difference of end and start values for this model. I'd prefer not to use an extra column or another patchy solution. What is the django-ish way of doing this?</p>