Set up a Timer, and set the Form.Text property in the handler:
Form Constructor code:
Timer showTime = new Timer();
showTime.Interval = 500;
showTime.Tick += new EventHandler(showTime_Tick);
showTime.Start();
Handler:
void showTime_Tick(object sender, EventArgs e)
{
Text = DateTime.Now.ToString("HH:mm:ss");
}