coundown timer

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

function start_onload(last_hour){
    var timeout_message = document.getElementById('timeout_message');
    var currentTime = new Date();
    var hours = currentTime.getHours();
    var minutes = currentTime.getMinutes();
    var seconds = currentTime.getSeconds();
    var expire_time = 0; // in seconds
    if (hours<last_hour) {
        expire_time += (last_hour-hours-1)*3600;
        expire_time += (59-minutes)*60;
        expire_time += (59-seconds);
    }
    else {
        timeout_message.innerHTML = 'It\'s after '+last_hour+' o\'clock!';
        return;
    }
    var expire_time = currentTime.getTime() + 1000*expire_time;
    //console.log(expire_time, hours, minutes, seconds, expire_time);
    function countdown_session_timeout() {
        var current_time = new Date().getTime();
        var remaining = Math.floor((expire_time - current_time)/1000);
        if (remaining>0) {
            hours = Math.floor(remaining/3600);
            minutes = Math.floor((remaining - hours*3600)/60);
            seconds = remaining%60;
            timeout_message.innerHTML = 'Countdown will stop in '+ hours + ' hours ' + minutes + ' min. ' + seconds + ' sec.';
            setTimeout(countdown_session_timeout, 1000);
        } else {
            timeout_message.innerHTML = 'Time is up!';
        }
    }
    countdown_session_timeout();
}

</script>
</head>
<body onload="start_onload(18)">
<h1> Countdown example </h1>
<p> http://stackoverflow.com/questions/11548276/how-do-i-create-a-countdown-time-that-resets-at-a-specific-time </p>
<div id="timeout_message" style="position:absolute; left:0; bottom:0; background-color: #ddd; color: red;"></div>
</body>