Calculate difference between zero and window.onload

ev.onload.js

    <script>
        // calculate difference between zero and window.onload        
        window.onload = function () {
            var stop = new Date();
            document.getElementById("content").style.display = "block";
            document.getElementById("ms").innerHTML          = (stop - start);
        };    
    </script>        
    ...
    <script>
    /* before script tags add this line: */
    var start = new Date();
    </script>