On a slightly different note, you definitely do not want this:
created_on = models.DateTimeField(…, default=datetime.datetime.utcnow())
This actually calls utcnow() when your models.py file is loaded (which will happen when you manage.py runserver) and use that single point in time for the default value for every instance there-after. What you want is this:
created_on = models.DateTimeField(…, default=datetime.datetime.utcnow)
The default argument can accept a callable, which will be called each time the default value is needed for a new instance. Have a look at the documentation.