timerID      = null;
timerRunning = false;

function stopclock()
{
    if(timerRunning)
        clearTimeout(timerID)
    timerRunning = false;
}

function startclock()
{
    stopclock();
    showtime();
}

function showtime()
{
    var now = new Date();
    var hours = now.getHours();
    var minutes = now.getMinutes();
    var seconds = now.getSeconds();
    var timeValue = "" + hours;
    timeValue  += ((minutes < 10) ? ":0" : ":") + minutes;
    timeValue  += ((seconds < 10) ? ":0" : ":") + seconds;
    //timeValue  += (hours >= 12) ? " P.M." : " A.M.";
    document.clock.face.value = timeValue;
    timerID = setTimeout("showtime()",1000);
    timerRunning = true;
}