Timer

Am I doing timer correctly? When I click Start, Milliseconds seems not run as fast. It runs like 1, 15, 35, 65 ,etc. It does not start from 1, 2, 3 (real fast here)
 DispatcherTimer tmr = new DispatcherTimer();
        System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();

            tmr.Interval = TimeSpan.FromSeconds(1);
            tmr.Tick += onTimerTick;
            tmr.Start();

void onTimerTick(object sender, EventArgs args)
        {
            textBox1.Text = string.Format("{0:D2}:{1:D2}:{2:D2}:{3:D3}",
                  sw.Elapsed.Hours,
                  sw.Elapsed.Minutes,
                  sw.Elapsed.Seconds,
                  sw.Elapsed.Milliseconds);
        }

        private void button2_Click(object sender, RoutedEventArgs e)
        {
            tmr.Stop();
            sw.Reset();
        }