What you have to do is, convert your time into seconds.
<?php
list($hour,$min,$sec) = explode(':', $dbSessionDuration);
$dbSessionDurationTime = mktime(0,0,0,$hour,$min,$sec);
?>
To create a countdown, use Javascript.
<script type="text/javascript">
    var millis = <?php echo $dbSessionDurationTime; ?>
    function displaytimer(){
        var hours = Math.floor(millis / 36e5),
            mins = Math.floor((millis % 36e5) / 6e4),
            secs = Math.floor((millis % 6e4) / 1000);
            //Here, the DOM that the timer will appear using jQuery
            $('.count').html(hours+':'+mins+':'+secs);  
    }
    setInterval(function(){
        millis -= 1000;
        displaytimer();
    }, 1000);
</script>
I hope this helps you.