Session Timeout Warning

<%
Session.Timeout = 5 ' set to 5 minutes for example (we really are going to use 2 minutes 10 seconds)
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">

<head>
<title>Client Side Session Timeout Check</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta name="author" content="Roderick Divilbiss">
<meta name="copyright" content="© 2005-2010 Roderick Divilbiss">
<style type="text/css">
#warningMsg {
    position:absolute;
    display:none;
    width:375px;
    color:#000000;
    border:3px solid #FF0000;
    padding:10px;
    background-color:#FFFF00;
    top:-400px;
    left:-400px; font-weight:bold
}

#btnDiv {
    text-align:center;
}
</style>
<script type="text/javascript">
function popMessage() {
    var top=0;
    var left=0;
    var scrollTop = 0;

    var top;
    document.getElementById('warningMsg').style.display='block';
    scrollTop = (document.documentElement.scrollTop) ? document.documentElement.scrollTop : window.pageYOffset;
    // if top is set at the clientHeight minus the division height divided by 2 plus the scrollTop it will be in the middle;
    top = ((document.documentElement.clientHeight-82)/2)+scrollTop;
    // if left = the clientWidth minus the division width divided by two is the division will center.
    left = (document.documentElement.clientWidth-375)/2;
    document.getElementById('warningMsg').style.top=(top)+'px';
    document.getElementById('warningMsg').style.left=(left)+'px';
}

function closePopMessage(answer) {
    if (answer==true) {
        document.getElementById('warningMsg').style.display='none';
        top.location=document.location;
    }
}


var timeout=130000; // 2 minutes, 10 seconds...don't need to wait all day for an example
var warning=120000; // 2 minute warning
var dt = new Date();
var start=dt.getTime();
var warned=false;

function checkTime() {
    var d=new Date();
    var now=d.getTime();
    var timeSpent=(now-start);

    var secondsLeft=((timeout-timeSpent)/1000);
    var minutesLeft=Math.floor(secondsLeft/60);
    secondsLeft=Math.floor(secondsLeft-(minutesLeft*60));
    var refresh;
    if (timeSpent > timeout) {
        document.location='sessionExpired.asp';
    }
    if (timeSpent > (timeout-warning)) {
        if (warned==false) {
            popMessage();
            warned=true; 
        }
        document.getElementById('sessionStatus').innerHTML='You have in '+minutesLeft+' minutes: '+secondsLeft+' seconds remaining.';
    } else {
        document.getElementById('sessionStatus').innerHTML='You have in '+minutesLeft+' minutes: '+secondsLeft+' seconds remaining.';
    }
    setTimeout('checkTime()',1000);
}
</script>
</head>

<body onload="checkTime();">
<div><span id="sessionStatus"></span></div>

<div id="warningMsg">Your session will time out in two minutes.<br>
To keep your session active, please click OK.<br>
<div id="btnDiv"><input type="button" name="btnOK" value="OK" onclick="closePopMessage(true);"></div></div>
</body>

</html>
How can you warn a user that his ASP session is about to timeout?
