protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {

            UpdateTime();

        }

    }
    protected void Timer1_Tick(object sender, EventArgs e)
    {
        UpdateTime();
    }
    public void UpdateTime()
    {
        try
        {
            int hour = 00, min = 00, sec = Int32.Parse(Session["tleft"].ToString());
            // min = Int32.Parse(Session["tleft"].ToString()) / 
            TimeSpan span = new TimeSpan(TimeSpan.TicksPerSecond * sec);
            string str = span.Hours.ToString("00") + ":" + span.Minutes.ToString("00") + ":" + span.Seconds.ToString("00");

            int tleft = Int32.Parse(Session["tleft"].ToString()) - 1;
            if (tleft <= 0)
            {
                Response.Redirect("result.aspx");
            }

            // timeleft.Text = tleft.ToString();
            timeleft.Text = str.ToString();
            Session["tleft"] = tleft.ToString();
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message.ToString());
        }
    }

    -----------------------------------------------------------------------

    <%@ Page Language="C#" AutoEventWireup="true" CodeFile="TickTock.aspx.cs" Inherits="TickTock" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" >
    </asp:ScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
    <div>
        <asp:Label ID="timeleft" runat="server" ></asp:Label>
        <asp:Timer ID="Timer1" Enabled="true"  runat="server" Interval="1000" 
            ontick="Timer1_Tick" >
        </asp:Timer>
    </div>

    </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="Timer1" />
        </Triggers>
    </asp:UpdatePanel>
    </form>
</body>
</html>

You need to provide more information. What isn't working? What error messages to you see? Are you sure the "online" environment is supporting ASP.NET?

yes online environment is supporting asp.net 2008. it doesnot show any error. please check http://www.testuonline.com. I dont know why timer is not ticking when i upload it on the online server. but timer is ticking fine when the page runs offline on my system.