Time Difference

 Hi, I want to subtract textbox2 from textbox3 to get the duration of my transaction .
Heres what I using to find the exact time
TextBox2.Text = DateTime.Now.ToLongTimeString() + ":" + DateTime.Now.Millisecond.ToString();   TextBox3.Text = DateTime.Now.ToLongTimeString() + ":" + DateTime.Now.Millisecond.ToString(); How do I take away these times to show in textbox4