<!DOCTYPE html>
<html>
<body>
<p>A script on this page starts this clock:</p>
<p id="demo"></p>
<button onclick="clearInterval(myVar)">Stop time</button>
<script>
var myVar = setInterval(myTimer ,1000);
function myTimer() {
var d = new Date();
document.getElementById("demo").innerHTML = d.toLocaleTimeString();
}
</script>
</body>
</html>