How to use Stopwatch

using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Diagnostics;
using System.Globalization;
using System.Drawing.Drawing2D;
using System.Runtime.InteropServices;

public partial class _Default : System.Web.UI.Page
{
    Stopwatch sw=new Stopwatch ();
    bool pas = false;
    protected void Page_Load(object sender, EventArgs e)
    {

        sw.Start();
    }

    

    protected void Timer1_Tick(object sender, EventArgs e)
    {
        if (sw.IsRunning)
        {
            TimeSpan objTimeSpan = TimeSpan.FromMilliseconds(sw.ElapsedMilliseconds);
            Label1.Text = String.Format(CultureInfo.CurrentCulture, "{0:00}:{1:00}:{2:00}.{3:00}", objTimeSpan.Hours, objTimeSpan.Minutes, objTimeSpan.Seconds, objTimeSpan.Milliseconds / 10);

            if (pas)
            {
                pas = false;
            }
        }

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        sw.Start();
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        sw.Stop();
    }

    protected void Button3_Click1(object sender, EventArgs e)
    {
        Label1.Text = "00:00:00.00";
        sw.Reset();
    }
}