Windows phone的递增计时器

我在Windows手机上制作计时器应用程序时有问题。我有文本框设置为00:00:00，我试图增加它每秒，但第一秒后，它不会做任何更多。我相信这是一个容易的解决方案，并将非常感谢任何帮助。谢谢你

公共主页(){

InitializeComponent ();

        DispatcherTimer timer = new DispatcherTimer();
        timer.Interval = TimeSpan.FromSeconds(1);
        timer.Tick += OnTimerTick;
        timer.Start();
    }
    void OnTimerTick(object sender, EventArgs args)
    {
        txtTimer.Text = DateTime.Now.ToString();
    }
    private void btnStartClick(object sender, EventArgs e)
    {
        DispatcherTimer timer = new DispatcherTimer();
        timer.Tick +=
            delegate(object s, EventArgs args)
            {
                TimeSpan time = new TimeSpan(0);
                time += TimeSpan.FromSeconds(1);                   
                this.timenow.Text = string.Format("{0:D2}:{1:D2}:{2:D2}", time.Hours, time.Minutes, time.Seconds);
            };

        timer.Interval = new TimeSpan(0, 0, 1); 
        timer.Start();
    }

            TimeSpan time = new TimeSpan(0);
            time += TimeSpan.FromSeconds(1);                   
            this.timenow.Text = string.Format("{0:D2}:{1:D2}:{2:D2}", time.Hours, time.Minutes, time.Seconds);