var timerID = null;
var timerRunning = false;

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

function showtime () {
   var now = new Date();
   var hours = now.getHours();
   var minutes = now.getMinutes();
   var seconds = now.getSeconds();
   var timeValue = "";
   timeValue += ((hours < 10) ? "0" : "") + hours;
   timeValue += ((minutes < 10) ? ":0" : ":") + minutes;
   timeValue += ((seconds < 10) ? ":0" : ":") + seconds;
   document.all.timer.innerHTML = timeValue;
   timerID = setTimeout("showtime()",1000);
   timerRunning = true;
}

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