Well, the above answer is correct, auto_now_add and auto_now would do it, but it would be better to make an abstract class and use it in any model where you require created_at and updated_at fields.
class TimeStampMixin(models.Model):
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
class Meta:
abstract = True
Now anywhere you want to use it you can do a simple inherit and you can use timestamp in any model you make like.
class Posts(TimeStampMixin):
name = models.CharField(max_length=50)
...
...
In this way, you can leverage object-oriented reusability, in Django DRY(don't repeat yourself)