Difference between auto_now and auto_now_add
These fields are built into Django for expressly this purpose — auto_now fields are updated to the current timestamp every time an object is saved and are therefore perfect for tracking when an object was last modified, while an auto_now_add field is saved as the current timestamp when a row is first added to the database and is therefore perfect for tracking when it was created.
What I understood in Django models field's attributes is  

auto_now - updates the value of field to current time and date every time the Model.save() is called.
auto_now_add - updates the value with the time and date of creation of record.

My question is what if a filed in model contains both the auto_now and auto_now_add set to True? What happens in that case?