How do I...How to create a counter in Asp.Net?

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:Timer ID="Timer1" runat="server" Interval="1000" ontick="Timer1_Tick">
        </asp:Timer>
        <asp:Label ID="Label1" runat="server" Text=""></asp:Label>
    </ContentTemplate>
</asp:UpdatePanel>

protected void Page_Load(object sender, EventArgs e)
{
    if (Session["CountdownTimer"] == null)
    {
        Session["CountdownTimer"] = new CountDownTimer(TimeSpan.Parse("2:30:00"));
        (Session["CountdownTimer"] as CountDownTimer).Start();
    }
}
protected void Timer1_Tick(object sender, EventArgs e)
{
    if (Session["CountdownTimer"] != null)
    {
        Label1.Text = (Session["CountdownTimer"] as CountDownTimer).TimeLeft.ToString();
    }
}

public class CountDownTimer
{
    public TimeSpan TimeLeft;
    System.Threading.Thread thread;
    public CountDownTimer(TimeSpan original)
    {
        this.TimeLeft = original;
    }
    public void Start()
    {
        // Start a background thread to count down time
        thread = new System.Threading.Thread(() =>
        {
            while (true)
            {
                System.Threading.Thread.Sleep(1000);
                TimeLeft = TimeLeft.Subtract(TimeSpan.Parse("00:00:01"));
            }
        });
        thread.Start();
    }
}