title: How to add a given number of days to a django DateTimeField
body: <p>I have a model with a field containing a DateTimeField and a function to calculate a return date based on this DateTimeField. How would I add a given number of days to this????</p>
  <pre><code>class MyModel(models.Model):
      startdate = models.DateTimeField(auto_now_add=True)
      enddate = models.DateField()
  
      def save(self)
          self.enddate = self.startdate + datetime.timedelta(days=5)
          super(MyModel, self).save(*args, **kwargs)
  </code></pre>
  <p>This however does not work</p>