当时间达到零时显示弹出窗口？

Question

Timer.aspx

<div>
        <asp:ScriptManager ID="SM1" runat="server">
        </asp:ScriptManager>
        <asp:Timer ID="timer1" runat="server" Interval="1000" OnTick="timer1_tick">
        </asp:Timer>
    </div>
    <div>
        <asp:UpdatePanel ID="updPnl" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                <asp:Label ID="lblTimer" runat="server"></asp:Label>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="timer1" EventName="tick" />
            </Triggers>
        </asp:UpdatePanel>
    </div>

代码隐藏文件 Timer.aspx.cs

public partial class Timer : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

        if (!SM1.IsInAsyncPostBack)
            Session["timeout"] = DateTime.Now.AddMinutes(11).ToString();

    }
    protected void timer1_tick(object sender, EventArgs e)
    {
        if (0 > DateTime.Compare(DateTime.Now,
        DateTime.Parse(Session["timeout"].ToString())))
        {
            lblTimer.Text = "Time Left: " +
            ((Int32)DateTime.Parse(Session["timeout"].
            ToString()).Subtract(DateTime.Now).TotalMinutes).ToString()+"  minutes";
        }
    }
}

以下是Asp.net AJAX中10分钟计时器的代码。我在哪里编写弹出窗口的代码，当时间达到零时显示？