Setting auto_now and auto_now_add to True will automatically set a DateField or a DateTimeField to the current date and time. This works great for "created" and "modified" fields.
However, suppose you're importing content from another system, and you want to maintain the existing timestamps. If you're using auto_now..., then it won't work--you can't override them. Ouch.
Instead, set default to the timezone.now or date.today function.
from datetime import date
from django.utils import timezone # <-- you need Django's timezone module
created = models.DateTimeField(default=timezone.now)
created = models.DateField(default=date.today)
See the Django docs.