C# Silverlight childwindow在时间戳结束后关闭

我有一个silverlight应用程序，我想在30秒后自动关闭我的孩子窗口，但我不知道如何使用这种时间戳。

您的孩子窗口是ChildWindow1，请执行以下操作

    DispatcherTimer idleTimer;
    DateTime timeNow;
    public ChildWindow1()
    {
        InitializeComponent();

        idleTimer = new DispatcherTimer();
        idleTimer.Start();
        idleTimer.Interval = TimeSpan.FromSeconds(1);

        idleTimer.Tick += new EventHandler(idleTimer_Tick);
        timeNow= DateTime.Now;
        // Initialise last activity time
    }

    private void OKButton_Click(object sender, RoutedEventArgs e)
    {
        this.DialogResult = true;
    }

    private void CancelButton_Click(object sender, RoutedEventArgs e)
    {
        this.DialogResult = false;
    }
    private void idleTimer_Tick(object sender, EventArgs e)
    {
        if (DateTime.Now > timeNow.AddSeconds(30))
        {
            this.Close();
        }
    }