You have set the Login Time in TextBox one as
TextBox1.Text = DateTime.Now.ToString();
and Logout time in TextBox2 as
TextBox2.Text = DateTime.Now.ToString();
Now if you want to find the Time difference between these two dates then please refer this code.
TimeSpan timeSpan = Convert.ToDateTime(TextBox2.Text) - Convert.ToDateTime(TextBox1.Text);
TextBox3.Text = string.Format("Hours: {0} Minutes: {1} Seconds: {2}", timeSpan.Hours, timeSpan.Minutes, timeSpan.Seconds);