How can my django model DateField add 30 days to the provided value?

Use Pythons timedelta:

from datetime import timedelta
d = timedelta(days=30)

# object is your current instance of the model
object.yourdatefield += d
# or this because I am not sure whether the previous works
object.yourdatefield = object.yourdatefield + d

object.save()

And from the Django documentation:

DateField
A date, represented in Python by a datetime.date instance.

If you want to add 30 days on creation of the object, forget about auto_now_add=True and do as becomingGuru suggests. Information about overriding save() can also be found in the Django documentation.