Web Forms :: Display Session (Timeout) Expire Message 5 Seconds Before Session Expire

Using this code i want to show an modal pop up to the user that "your session will be expired within 5 minutes , Click here [BUTTON] to reset your session" , here's my code :

<asp:Button ID="btnReset" Text="Reset" runat="server" OnClick="ResetSession" />
<br />
Your Session will expire in <span id = "seconds"></span> seconds.
<script type="text/javascript">
function SessionExpireAlert(timeout) {
var seconds = timeout / 1000;
seconds--;

[CODE]...