Timeout message before session expire

Here a javascript solution for detecting session expire.

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title></title>
 
<style type="text/css">
        .message
        {
            border: 1px solid #CCCCCC;
            position: absolute;
            width: 400px;
            border: 1px solid #c93;
            background: #ffc;
            padding: 5px;
            left: 0px;
            top: -170px;
        }
    </style>    
<script language="javascript" type="text/javascript">
        function ShowAlert() {
            //get the height and width of the screen
            var windowWidth = document.documentElement.clientWidth;
            var windowHeight = document.documentElement.clientHeight;

            //get the height and width of the message box
            var popupHeight = $("#TimeoutMessage").height();
            var popupWidth = $("#TimeoutMessage").width();

            //show the message box
            $("#TimeoutMessage").animate({
                "top": windowHeight / 2 - popupHeight / 2,
                "left": windowWidth / 2 - popupWidth / 2
            }, 1000);

            //close the message box when cross red image is clicked
            $("#close_message").click(function() {
                $("#TimeoutMessage").fadeOut("slow");
            });

            setTimeout('Redirect()', 300000); //15 minutes in milliseconds
        }

        function Redirect() {
            window.location = "../../logout.aspx";
        }

    </script>
  
</head>
<body>
    <form id="form1" runat="server">
     <div id="TimeoutMessage" class="message">

        <p>
            You will be logged out due to inactivity in 5 minutes.<br /><br />
            Please refresh this page, or click on an action to refresh your session.</p>
    </div>
 
    </form>
    <script language="javascript" type="text/javascript">
        $(document).ready(function () {
            setTimeout('ShowAlert()', <%=((Session.Timeout * 60) * 1000) - 300000 %>);
        });
    </script>
</body>
</html>