WPF

 public Class ST
{
   private readonly System.Timers.Timer _timer;
   private DateTime _lastScanTime;
  
 public ST()
        {
            
            _timer = new System.Timers.Timer();
            _timer.Interval = 250;
            _timer.Start();
            _timer.Elapsed += OnTimerElapsed;
        }



         public TimeSpan ScanTime { get; private set; }

          private void OnTimerElapsed(object sender, ElapsedEventArgs e)
        {
            try
            {
                _timer.Stop();
                ScanTime = DateTime.Now - _lastScanTime;
                
                OnValuesRefreshed();
            }
            finally
            {
                _timer.Start();
            }
            _lastScanTime = DateTime.Now;
        }
        
        private void OnValuesRefreshed()
        {
            ValuesRefreshed?.Invoke(this, new EventArgs());
        }
}

private void Window_Loaded(object sender, RoutedEventArgs e)
       {
           _timer = new System.Timers.Timer();
           _timer.Interval = 1000;
           _timer.Elapsed += OnTimerElapsed;
           _timer.Start();

       }

       int i=0;
       private void OnTimerElapsed(object sender, ElapsedEventArgs e)
       {
           i++;

           label1.Content = i.ToString();

       }

label1.Content = i.ToString();

<Label x:Name="label1"  HorizontalAlignment="Center" Margin="2" VerticalAlignment="Center" Height="79" Width="86"/>