How to deny access to URL when a countdown timer runs out

[mexican_hippo]

I am trying to make a countdown timer for a special offer.

It would work like this:

1. The timer starts when a person visits the landing page. On this landing page there is a link to an offer
2. When the timer runs then the person must be denied access to the offer
and that's it

Now I have already made a countdown timer, but I have no idea how to deny access to the link when it runs out.

Any suggestions?

p.s. I'm not exactly a coding genius so please try to simplify