How to subtract the date and time with multiple-choice values


I am thinking of a way of calculating the date by subtracting values in multiple choice filed with current date

I have a model which has multiple choices with values that I want to subtract with but I am not sure how it should be done.

    (5,'5 minutes before'),
    (15, '15 minutes before'),
    (30, '30 minutes before'),
    (60, '1 hour before'),
    (120, '2 hours before'),
    (240, '4 hours before'),
    (1440, '1 day before'),

Just substract a timedelta like this:

date_b = date_a - datetime.timedelta(minutes=30)


date_b = date_a - datetime.timedelta(days=2, minutes=30)