Javascript Timer

Hi all.  I have a java script timer that executes on a slide.  When I replay the screen, I need it to reset the timer.  What happens right now, is that it seems to add a new timer along with the previous timer, and they interfere with each other, given times from each timer back and forth.  Is there a way that I can clear the previous timer out when I replay this screen?  Here is the javascript that I am executing on the slide:

var player=GetPlayer();

function countdown( elementName, minutes, seconds )
{
    var endTime, hours, mins, msLeft, msTotal, time;

    function twoDigits( n )
    {
        return (n <= 9 ? "0" + n : n);
    }

    function updateTimer()
    {
        msLeft = endTime - (+new Date);
        if ( msLeft < 1000 ) {
            player.SetVar("timer", "0:00");

        } else {
            time = new Date( msLeft );
            hours = time.getUTCHours();
            mins = time.getUTCMinutes();
            player.SetVar("timer", (hours ? hours + ':' + twoDigits( mins ) : mins) + ':' + twoDigits( time.getUTCSeconds() ));
            setTimeout( updateTimer, time.getUTCMilliseconds() + 500 );
       }

    }

    endTime = (+new Date) + 1000 * (60*minutes + seconds) + 500;

    msTotal = endTime - (+new Date);

    updateTimer();
}
countdown( "timer", 4, 00 );