The proper way to achieve is to add instance method to the `DeliveryWindow` model and add an property for it is even better. 
https://docs.djangoproject.com/en/dev/topics/db/models/#model-methods

Make sure the naming make sense. 

For example, 
https://github.com/Fruitex/fruitex/blob/dev/order/models.py#L137
https://github.com/Fruitex/fruitex/blob/dev/order/models.py#L163