w3schools Window clearTimeout() Method

 <button onclick="startCount()">Start count!</button>
<input type="text" id="txt">
<button onclick="stopCount()">Stop count!</button>


 <script>
var c = 0;
var t;
var timer_is_on = 0;


 function timedCount() {
  document.getElementById("txt").value = c;

    c = c + 1;
  t = setTimeout(timedCount, 1000);
}

function startCount() {
  if (!timer_is_on) {

      timer_is_on = 1;
    timedCount();

    }
}

function stopCount() {
  clearTimeout(t);
  timer_is_on = 0;

 }
</script>