MainActivity.cs

using Android.App;
using Android.OS;
using Android.Support.V7.App;
using Android.Runtime;
using Android.Widget;
using System.Threading.Tasks;
using System;

namespace digitalClock
{
    [Activity(Label = "@string/app_name", Theme = "@style/AppTheme", MainLauncher = true)]
    public class MainActivity : AppCompatActivity
    {
        Task _timer;
        TextView nowTimeLabel;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.activity_main);

            nowTimeLabel = FindViewById<TextView>(Resource.Id.nowTimeLabel);

            _timer = new Task(async () =>
            {
                while(true)
                {
                    RunOnUiThread(() =>
                    {
                        nowTimeLabel.Text = DateTime.Now.ToString("HH:mm:ss");
                    });
                    await Task.Delay(1000);
                }
            });

            _timer.Start();
        }
    }
}