partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            clock = new Clock(digital);
        }
        private void start_Click(object sender, System.EventArgs e)
        {
            this.clock.Start();
        }
        private void stop_Click(object sender, System.EventArgs e)
        {
            this.clock.Stop();
        }
        private Clock clock;
    }

class Clock
	{
        public Clock(TextBox displayBox)
        {
            this.display = displayBox;
        }
        public void Start()
		{
            //pulsed.Add(this.RefreshTime);
            pulsed.tick += this.RefreshTime;
        }
		public void Stop()
		{
            //pulsed.Remove(this.RefreshTime);
            pulsed.tick -= this.RefreshTime;
        }
		private void RefreshTime(int hh, int mm, int ss)
		{
			this.display.Text = string.Format("{0:D2}:{1:D2}:{2:D2}", hh, mm, ss);
		}
		private Ticker pulsed = new Ticker();
        private TextBox display;
    }