Hora em tempo real - JavaScript

<!DOCTYPE html>
<html>
<body>
<p>Hora: <span id="demo"></span></p>
<script>
var myVar=setInterval(function(){myTimer()},1000);
function myTimer() {
    var d = new Date();
    document.getElementById("demo").innerHTML = d.toLocaleTimeString();
}
</script>
</body>
</html>

Demo: http://jsfiddle.net/5rkna6kd/