Calling javascript from the page load with out any event fire

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            Page.ClientScript.RegisterStartupScript(this.GetType(),"onLoad","DisplaySessionTimeout()", true);
        }
    }



<script type="text/javascript">
        var sessionTimeout = <%= Session.Timeout %>
       
        function DisplaySessionTimeout()
        {
            document.getElementById("<%= lblSessionTime.ClientID %>").innerText = sessionTimeout;
            sessionTimeout = sessionTimeout - 1;
           
            if (sessionTimeout >= 0)
                window.setTimeout("DisplaySessionTimeout()", 60000);
            else
            {
                alert("Your current Session is over.");
            }
        }
    </script>