JavaScript/WindowOrWorkerGlobalScope/clearTimeout

 var elem = document.getElementById('los');
 elem.addEventListener('click', startCountdown);	
 
 var elem = document.getElementById('stop');
 elem.addEventListener('click', stopCountdown);	
 function startCountdown () {
 countdown(10, 'counter');
 EnableCancelButton (true);
 }
 function stopCountdown() {
 var element = document.getElementById('counter');
 clearTimeout(element.timerId);
 EnableCancelButton(false);
 }
 function EnableCancelButton(enable) {
 var cancelButton = document.getElementById ("stop");
	cancelButton.disabled = !enable;
 }
 
 function leadingzero(number) {
 return (number < 10) ? '0' + number : number;
 }
 function countdown(seconds, target) {
 var element = document.getElementById(target);
 element.seconds = seconds;
 calculateAndShow(target);
 }
 function calculateAndShow(target) {
 var element = document.getElementById(target);
 if (element.seconds >= 0) {
 element.timerId = window.setTimeout(calculateAndShow,1000,target);
 var h = Math.floor(element.seconds / 3600);
 var m = Math.floor((element.seconds % 3600) / 60);
 var s = element.seconds % 60;
 element.innerHTML=
 leadingzero(h) + ':' +
 leadingzero(m) + ':' +
 leadingzero(s);
 element.seconds--;
 } else {
 completed(target);
 return false;
 }
 }
 function completed(target) {
 var element = document.getElementById(target);
 element.innerHTML = "<strong>Liftoff!<\/strong>";
 }