Linkning text to variabels

I am currently working on a project for a customer that sells coupons on their website.

It's possible for the visitor of the website to design their own coupons.

The customer would like to have the end date of the coupon on the back of the coupon, and my question is:

As how far is it possible to link the end date to this coupon (end date = production date + X amount of days)