How can I set a cookie to expire after x days with this code I have?

Possible Duplicate:
What is the “best” way to get and set a single cookie value using JavaScript

So I have this code (which I did not make) which basically makes a container with an X, so when I press the X, the div container closes. However, when you refresh the page, the div container will reappear again.

How can I make it so when the person presses the X button, the person will never see the div container ever again (or for a set amount of time like 30 days) on that page?

I want to take it one step further, if possible, and make it so when the person presses the X button, he/she will never see the div container again throughout my site, as I plan on implementing the same div container throughout my site.

Hope this isn't too confusing, and that someone can help me out. Thanks!

HTML Code:

<div id="bottom_ad">
    <div id="close_ad" onclick="close_bottom_ad();">X</div>
    <!-- Ad Content -->
</div>

CSS Code:

#bottom_ad
{
    position: absolute;
    right: 0;
    bottom: 0;
    left: 0;
    height: 80px;
    background: #000;
}
#close_ad
{
    position: absolute;
    top: 8px;
    right: 8px;
    width: 15px;
    height: 15px;
    color: #fff;
}

JavaScript (Place at bottom of file, before tag.) Code:

<script type="text/javascript">
    // I may have sorta kinda taken this function from W3Schools. :S
    function getCookie(c_name)
    {
        var i,x,y,ARRcookies=document.cookie.split(";");
        for (i=0;i<ARRcookies.length;i++)
        {
            x=ARRcookies[i].substr(0,ARRcookies[i].indexOf("="));
            y=ARRcookies[i].substr(ARRcookies[i].indexOf("=")+1);
            x=x.replace(/^\s+|\s+$/g,"");
            if (x==c_name)
            {
                return unescape(y);
            }
        }
    }

    var ad_cookie = getCookie("closedAd");

    if (ad_cookie != null && ad_cookie != "")
    {
        document.getElementById("bottom_ad").style.display="none";
    }

    function close_bottom_ad()
    {
        document.getElementById("bottom_ad").style.display="none";
        document.cookie = "closedAd=true;";
    }
</script>