title: Django diferrent value of DateTimeField at created/updated
body: <p>I would like to store time of created and modified. But when I use below model:</p>
  
  <pre><code>date_created = models.DateTimeField(auto_now_add=True)
  date_update_local = models.DateTimeField(auto_now=True)
  </code></pre>
  
  <p><code>auto_now_add</code> does't work properly. If I created object at first time, everything is ok, I have properly value of <code>date_created</code>, but if I modify exist object then <code>date_created</code> is set to <code>0000-00-00 00:00:00.000000</code>. </p>
  
  <p>So I modified my code as below:</p>
  
  <p>Models.py</p>
  
  <pre><code>class Product(models.Model):
      reference = models.CharField(max_length=8, unique=True, primary_key=True)
      date_created = models.DateTimeField(default=timezone.now)
      date_update_local = models.DateTimeField(auto_now=True)
  
      def save(self, *args, **kwargs):
          try:
              p = Product.objects.get(reference=self.reference)
              self.date_created = p.date_created
          except self.DoesNotExist:
              pass
          return super(Product, self).save(*args, **kwargs)
  </code></pre>
  
  <p>Views.py</p>
  
  <pre><code>p = Product(reference = 'no. ref')
  p.save()
  </code></pre>
  
  <p>I am checking does object exist, if True, I am get last <code>date_created</code> value and set it as new value. But I think it is wrong solution.</p>