How to add a given number of days to a django DateTimeField

Question

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????

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)

This however does not work