C#

DateTime dt1 = DateTime.ParseExact("11:15:00", "hh:mm:ss", new DateTimeFormatInfo());
DateTime dt2 = DateTime.ParseExact("11:20:00", "hh:mm:ss", new DateTimeFormatInfo());

TimeSpan ts1 = dt1.Subtract(dt2);
Console.WriteLine(ts1.ToString());

private DateTime started;

public Form1() {
    InitializeComponent();
    started=DateTime.Now;
    timer.Tick += new EventHandler(timer1_Tick);
    timer.Interval = (1000) * (1);
    timer.Start();
}

private void timer1_Tick(object sender, EventArgs e) {
    DateTime now=DateTime.Now;
    label6.Text = "It is now "+now.ToString("HH:mm:ss");
    double seconds=now.Subtract(started).TotalSeconds;
    label7.Text = "which is "+seconds+" seconds since this form started.";
}