brad24.html

<script>
let counter = 0;
let timer;

function go(){
    timer = setInterval(doThing, 1*1000);
}
function doThing(){
    document.getElementById('counter').innerHTML = counter++;
}
function stopGame(){
    clearInterval(timer);
}

</script>
Timer : <span id='counter'></span>
<hr>
<input type="button" onclick="go()" value='Go!'/>
<input type="button" onclick="stopGame()" value="Stop" />