Tryit Editor v2.7

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<!DOCTYPE html>
<html>
<body>

<p>Click the button to display the UTC time.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function addZero(x,n) {
    while (x.toString().length < n) {
        x = "0" + x;
    }
    return x;
}

function myFunction() {
    var d = new Date();
    var x = document.getElementById("demo");
    var h = addZero(d.getUTCHours(), 2);
    var m = addZero(d.getUTCMinutes(), 2);
    var s = addZero(d.getUTCSeconds(), 2);
    var ms = addZero(d.getUTCMilliseconds(), 3);
    x.innerHTML = h + ":" + m + ":" + s + ":" + ms;
}
</script>

</body>
</html>