To set a 30 day expiration on your cookie, you need to add an expiration time to the cookie. Here's a pretty simple cookie function that sets a cookie for N days:
function createCookie(name, value, days) {
var date, expires;
if (days) {
date = new Date();
date.setTime(date.getTime()+(days*24*60*60*1000));
expires = "; expires="+date.toUTCString();
} else {
expires = "";
}
document.cookie = name+"="+value+expires+"; path=/";
}
By setting an appropriate expiration date in the cookie, it will be removed automatically at that time.
Then, when your page loads (you will have to wait until the page has finished loading before attempting to modify it), you can check to see if your cookie closeAd is set and if so, execute the code to hide the ad. For a more flicker free viewing experience (so the ad doesn't first show open, then close), you can either start out with the ad hidden and only show it if there is no cookie. Or you can only dynamically add it to the page if there is no cookie.
