Countdown Timer in Xamain.Forms App

I am trying to convert Xamarin.Android app to Xamarin.Forms, I nedd to have a countdown timer for exam format which will keep updating a label while we use the app. Once timer goes off message is displayed and another activity is opened. I had used follwoing code for Xamarin.Android app.

Can anyone help me understand how i can add this functionality to xamarin.forms app.

   public delegate void TickEvent(long millisUntilFinished);
     public delegate void FinishEvent();

    public class CountDown : CountDownTimer
    {
        public event TickEvent Tick;
        public event FinishEvent Finish;

        Activity m_owner;
        public CountDown(long totaltime, long interval, Activity owner): base(totaltime, interval)
        {
                m_owner = owner;
        }
        public override void OnTick(long millisUntilFinished)
        {
            if (Tick != null)
            Tick(millisUntilFinished);
                long millis = millisUntilFinished;
                long hours = millis / (60 * 60 * 1000);
                long min = (millis / (60 * 1000)) % 60;
                long sec = (millis / 1000) % 60;
                TextView txtTime = m_owner.FindViewById<TextView>(Resource.Id.textTime);
                txtTime.Text = hours.ToString("00") + ":" + min.ToString("00") + ":" + sec.ToString("00");
        }
        public override void OnFinish()
        {
            var bulider = new AlertDialog.Builder(m_owner);
            bulider.SetMessage("Exam Time over");
            bulider.SetCancelable(false);
            bulider.SetPositiveButton("OK", delegate { });
            var dialog1 = bulider.Create();
            dialog1.Show();
            int res = 0;

            //start another activity
            m_owner.StartActivity (intent);

            if (Finish != null)
                Finish();
    }