session expire time countdown in javascript

<?php session_start();
// http://www.daniweb.com/web-development/php/threads/124500/session-time-out-in-php
// set timeout period in seconds
$inactive = 600;

// check to see if $_SESSION['timeout'] is set
if(isset($_SESSION['timeout']) ) {
    $session_life = time() - $_SESSION['start'];
    if($session_life > $inactive)
        { session_destroy(); header("Location: logoutpage.php"); }
}
$_SESSION['timeout'] = time();

?>
<html>
<head>
<script type="text/javascript">

function start_onload(){
    var expire_time = new Date().getTime() + 1000*<?php echo $inactive; ?>;
    //console.log(expire_time);
    countdown_session_timeout();
    function countdown_session_timeout() {
        var current_time = new Date().getTime();
        var remaining = Math.floor((expire_time - current_time)/1000);
        var timeout_message = document.getElementById('timeout_message');
        if (remaining>0) {
            timeout_message.innerHTML = 'Session will expire in '+ Math.floor(remaining/60) + ' min. ' + (remaining%60) + ' sec.';
            setTimeout(countdown_session_timeout, 1000);
        } else {
            timeout_message.innerHTML = 'Session expired.';
        }
    }
}

</script>
</head>
<body onload="start_onload()">
<h1>Example of how can be set timeout done</h1>
<p> http://stackoverflow.com/questions/11531299/html-session-timeout-in-progress-bar </p>
<div id="timeout_message" style="position:absolute; left:0; bottom:0; background-color: #ddd; color: red;"></div>
</body>