Help counting time down...

hello... i am working on a simple media player in a 2012 VS C# WPF project. The media element works fine... a little glitchy but i'm working on that, but i am having problems with my count down timer. i'm not sure how to get it into the ("HH:mm:ss") format... I've tried different ways and i'm not making progress and i'm not sure why... here's what i have:

using system.windows.threading;

public partial class MainWindow : Window
    {
        DispatcherTimer timer;
        int countDwn;

        public MainWindow()
        {
            InitializeComponent();
            timer = new DispatcherTimer();
            timer.Tick += new EventHandler(timer_Tick);
            timer.Interval = new TimeSpan(0, 0, 1);
            timer.Start();
        }
       
        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            
        }

        private void myMediaElement_MediaOpened(object sender, RoutedEventArgs e)
        {
            TimeSpan ts = myMediaElement.NaturalDuration.TimeSpan;
            sliderSeek.Maximum = ts.TotalSeconds;
            timer.Start();
        }

        private void sliderSeek_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            myMediaElement.Position = TimeSpan.FromSeconds(sliderSeek.Value);
        }

        void timer_Tick(object sender, EventArgs e)
        {
            sliderSeek.Value = myMediaElement.Position.TotalSeconds;
            countDwn = (int)sliderSeek.Maximum - (int)sliderSeek.Value;
            cntDwnLbl.Content = countDwn;
        }

it counts down correctly but it's in seconds. i can't seem to get it in "HH:mm:ss" format, i've tried various way and read different forum posts but haven't resolved this issue yet. any help would be great. thanks.

Adam