Demo of Timer

Design

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Watch
{
    public partial class Form1 : Form
    {
        int a, b, c;

        public Form1()
        {
            InitializeComponent();
        }

        private void timer1_Tick(object sender, EventArgs e)
        { 
            if(c > 100)
            {
                b++;
                c = 0;
            }
            if (b > 60)
            {
                a++;
                b = 0;
            }
            c++;

            label1.Text = a.ToString();
            label2.Text = b.ToString();
            label3.Text = c.ToString();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            timer1.Enabled = true;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            timer1.Enabled = false;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            a = b = c = 0;
        }

        private void button4_Click(object sender, EventArgs e)
        {
            listBox1.Items.Add(label1.Text + ":" + label2.Text + ":" + label3.Text);
        }

        private void button5_Click(object sender, EventArgs e)
        {
            listBox1.Items.Remove(listBox1.Text);
        }
    }
}

Output