When Django models field is empty, set value to the Default value

Whenever the user doesn't add a value, I need my Django models to replace the otherwise empty field with the value set in default.
My models looks like this:

not_before = models.TimeField(blank=True, null=True, default='00:00:00')
max_num_per_day = models.IntegerField(blank=True, null=True, default=0)

I tried every combination of null, blank and default but no matter what I do, the fields gets replaced by null instead of '00:00:00' and 0.

Is there anyway I can force it to the default value whenever the field is empty?