Popup via cookie with timer?

[bluesoul]

I know basically how to lay this out but the details are a little strange. I want to pop up an ad on the first page on the forums that a user visits, and once it's popped up set a timer in the user's cookie so they won't see another one for 12 hours. I could see dropping the popup code in the header so it'll be guaranteed to appear the first time but I don't do much manipulation of cookies so any advice on how to go about this would be helpful.

Thanks.