You definitely don't have to use the pass today from my experience.
You basically "buy" the pass using the code, but instead of activating you can choose to save it for another time. I was even sent the usual confirmation emails, but instead of it saying £6.99 it said GPB 0.00.
I should add the link in the original post didn't work. It kept saying the page didn't exist. I was sent an email from Now TV about the offer and used the link from there instead.