title: Django Custom Save Model
body: <p>I'm trying to perform a basic date calculation in a save model in django, see below code:</p>
  
  <pre><code>  class Purchase(models.Model):
      purchase_date = models.DateField()
      purchase_place = models.CharField(verbose_name='Place of Purchase', max_length=255)
      purchaseCategory = models.ForeignKey(PurchaseCategory, verbose_name='Purchase Category')
      cost = models.DecimalField(max_digits=11, decimal_places=2)
      warranty_period_number = models.IntegerField()
      warranty_period_type = models.CharField(max_length=255, choices=(('m', 'Month(s)'), ('y', 'Year(s)')))
      warranty_end_date = models.DateField(editable=False)
      scan = models.CharField(max_length=255)
      alerts = models.BooleanField(verbose_name='Receive Email Alerts?')
      user = models.ForeignKey('auth.User', editable=False)
      created = models.DateTimeField(editable=False, auto_now_add=True)
      modified = models.DateTimeField(editable=False, auto_now=True)
  
      #custom save model
      def save(self, *args, **kwargs):
          #figure out warranty end date
          if self.warranty_period_type == 'm':
              self.warranty_end_date = self.purchase_date + self.purchase_date.timedelta(months=self.warranty_period_number)
          else:
              self.warranty_end_date = self.purchase_date + self.purchase_date.timedelta(years=self.warranty_period_number)
          super(Purchase, self).save()
  </code></pre>
  
  <p>I thought the following would work but no luck.. it errors with:</p>
  
  <pre><code>'datetime.date' object has no attribute 'timedelta'
  </code></pre>
  
  <p>Can anyone point in the right direction to do what I'm trying to do?</p>
  
  <p>Cheers,
  Ben</p>