How do I invoke an event handler on session timeout

private void CheckSessionTimeout()
{
string msgSession = "Warning: Within next 3 minutes, if you do not do anything, our system will redirect to the login page. Please save changed data.";
//time to remind, 3 minutes before session end
int int_MilliSecondsTimeReminder = (this.Session.Timeout * 60000) - 3 * 60000;
//time to redirect, 5 miliseconds before session end
int int_MilliSecondsTimeOut = (this.Session.Timeout * 60000) - 5;

string str_Script = @"
var myTimeReminder, myTimeOut;
clearTimeout(myTimeReminder);
clearTimeout(myTimeOut); " +
"var sessionTimeReminder = " + int_MilliSecondsTimeReminder.ToString() + "; " +
"var sessionTimeout = " + int_MilliSecondsTimeOut.ToString() + ";" +
"function doReminder(){ alert('" + msgSession + "'); }" +
"function doRedirect(){ window.location.href='login.aspx'; }" + @"
myTimeReminder=setTimeout('doReminder()', sessionTimeReminder);
myTimeOut=setTimeout('doRedirect()', sessionTimeout); ";

ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(),
"CheckSessionOut", str_Script, true);
}