Get Time Difference between two Date Times using JavaScript

<html>
    <body>
        <canvas id="newCanvas" width="150" height="100"
style="border:1px solid green;"></canvas>
        <script type="text/javascript">
            //Start Time
            var d1 = new Date();
            // Do your operations as example given for d1
            var canvas = document.getElementById("newCanvas");
            var context = canvas.getContext("2d");
            context.fillStyle = "green";
            context.fillRect(0,0,100,60);
            //End Time
            var d2 = new Date();
            //Time difference in milli seconds
            document.write("Your Operation took " + (d2.getTime() - d1.getTime()) + " milliseconds");
        </script>
    </body>
</html>