Get User Spent Time onunload In Asp.Net and javascript

Question

Hi, I need to get the time every user spent on the page and insert that information to database.

When the page loads I assign a start time to a javascript value. on body's onunload function I calculate the time difference between the start time and end time so that I can insert it to database.

I use script manager and EnablePageMethods is set to true. And in the javascript function that I call onunload I call a web method by using pagemethods.

It seems to work fine in Firefox. However in Chrome and in IE it does not work.

Does any of you have any idea, because I am really loosing it. Thanks in advance.

The Script Manager code is :

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
                        </asp:ScriptManager>

The js code is something like that :

function ExitPage() {
        try {
        var leavingTime = new Date();
        alert("entering: " + enteringTime);
        alert("exiting: " + leavingTime);
        var diff = leavingTime - enteringTime;
        alert("diff: " + diff);
        alert(window.location);

        }
        catch (Err) {
            alert("Before error:" + Err.Description);
        }

        try {

            alert("Before PageMethods Call");

            PageMethods.set_path('SomePage.aspx');
            PageMethods.ClosePage(diff,onSuccess,onFail);
        }
        catch (Err) {
            alert("Error In PageMethods Block:" + Err.Description);
        }

        alert("Before Return");
        return false;

    }

    function onSuccess() {
        alert("Success");
    }

    function onFail() {
        alert("Fail");
    }

C# Code is something like that :

[System.Web.Services.WebMethod]
        public static void ClosePage(int milliseconds)
        {
            try{
                // Insert millisecond to database...
            }
            catch
            { }
        }