adsense script to prevetn click bombing

ok i googled a bit for this.and found one.but my prob is i dont understand javascript so plz help me with this script...


<div id="banners" onclick="trackclick();">

<script language="JavaScript">

if ( clickCount > 2 ){
document.write('
<a href="your clickbank link or anything else">
<img src="anything replacing adsense ad">
</a>
');
}else{
document.write('
adsense code goes here
');
}

</script>
</div>


now javascript handler for this

<script language="JavaScript">

var clickCount = readCookie();
if ( clickCount == null ){
clickCount = 0;
}

function trackclick() {
clickCount = parseInt(clickCount)+1;
setCookie( clickCount, 2 );
return true;
}

function setCookie( cookieValue, nDays) {
var today = new Date();
var expire = new Date();
if (nDays==null || nDays==0) nDays=1;
expire.setTime(today.getTime() + 3600000*24*nDays);
document.cookie = "clicktrack="+escape(cookieValue)
+ ";expires="+expire.toGMTString();
}

function readCookie() {
var nameEQ = "clicktrack=";
var ca = document.cookie.split(';');
for(var i=0;i < ca.length;i++) {
var c = ca[i];
while (c.charAt(0)==' ') c = c.substring(1,c.length);
if (c.indexOf(nameEQ) == 0){
return c.substring(nameEQ.length,c.length);
}
}
return null;
}

</script>

now i really dont understand this ndays concept..when will the cookies actually expire.i mean what time frame ? 24 hours or what ? plz help me guys...i need to ensure this thing before using the script....thanks in advance warriors