<!DOCTYPE html>
<html>
<body>

<p>This is a text.
<ins id="myIns" datetime="2012-09-15T22:55:03Z">This is an inserted text.</ins>
</p>

<p>Click the button to return the value of the datetime attribute of the inserted text.</p>

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

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

<script>
function myFunction() {
    var x = document.getElementById("myIns").dateTime;
    document.getElementById("demo").innerHTML=x;
}
</script>

</body>
</html>