Calculating speed using JavaScript returns NaN

Question

I am following the sample shown here How to detect internet speed in Javascript? where an image with known file size is downloaded from the web and the speed is determined. For some reason I am not getting a result. My implementation is slightly different in that I am calling the JavaScript using InvokeScript and getting the value via ScriptNotify. My result value, however, is NaN. What can I do to fix this?

JavaScript

var imageAddr = "http://www.tranquilmusic.ca/images/cats/Cat2.JPG";
    var startTime, endTime;
    var downloadSize = 5616998;
    var download = new Image();
    download.onload = function () {
        endTime = (new Date()).getTime();
        showResults();
    }
    startTime = (new Date()).getTime();
    download.src = imageAddr;

    function showResults() {
        var duration = (endTime - startTime) / 1000; //Math.round()
        var bitsLoaded = downloadSize * 8;
        var speedBps = (bitsLoaded / duration).toFixed(2);
        var speedKbps = (speedBps / 1024).toFixed(2);
        var speedMbps = (speedKbps / 1024).toFixed(2);

        window.external.notify("COT" + speedMbps);
    }

C#

private void RunTestButton_Click(object sender, RoutedEventArgs e)
    {
        object connectionType = Browser.InvokeScript("showResults");
    }

private void Browser_ScriptNotify(object sender, NotifyEventArgs e)
    {
        string value = null;
        value = e.Value.ToString();

        TempResultTextBlock.Text = value;
    }