How to set a Django model field's default value to a function call / callable (e.g., a date relative to the time of model object creation)

EDITED:

How can I set a Django field's default to a function that gets evaluated each time a new model object gets created?

I want to do something like the following, except that in this code, the code gets evaluated once and sets the default to the same date for each model object created, rather than evaluating the code each time a model object gets created:

from datetime import datetime, timedelta

class MyModel(models.Model):
    # default to 1 day from now
    my_datetime = models.DateTimeField(default=datetime.now() + timedelta(days=1))

ORIGINAL:

I want to create a default value for a function parameter such that it is dynamic and gets called and set each time the function is called. How can I do that? e.g.,

from datetime import datetime

def mydatetime(date=datetime.now()):
    print date

mydatetime() 
mydatetime() # prints the same thing as the previous call; but I want it to be a newer value

Specifically, I want to do it in Django, e.g.,

from datetime import datetime, timedelta

class MyModel(models.Model):
    # default to 1 day from now
    my_datetime = models.DateTimeField(default=datetime.now() + timedelta(days=1))