CODE:

using System;
using System.Windows.Forms;

namespace タイマー
{
    public partial class Form1 : Form
    {
      
        TimeSpan start = new TimeSpan(0, 0, 0);
        TimeSpan add = new TimeSpan(0, 0, 1);
        TimeSpan reset = new TimeSpan(0, 0, 0);
        bool bTimerFlag;

        public Form1()
        {
            InitializeComponent();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            start = start + add;
            label1.Text = start.ToString();
        }

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

        private void button2_Click(object sender, EventArgs e)
        {
            start = reset;
            timer1.Stop();
            if(bTimerFlag == false){
                label1.Text = start.ToString();
            }
            bTimerFlag = false;
        }

    }
}