how to calculate the time duration?

hi.. please check this code of mine:

protected void Button4_Click(object sender, EventArgs e)
{
DateTime dt1;
DateTime dt2;

string currenttime1 = DateTime.Now.ToLongTimeString();
string currenttime2 = DateTime.Now.ToLongTimeString();
TextBox2.Text = currenttime2;

dt1 = Convert.ToDateTime(currenttime1);
dt2 = Convert.ToDateTime(currenttime2);

TimeSpan ts = dt1 – dt2;

int hours = ts.Hours;
int min = ts.Minutes;

string finalMin = Convert.ToString(min);

TextBox3.Text = finalMin;

—- i’m having a web page that calculated time duration when the user press the stop button.. but the output of this code is always zero.. what is my error?
thanks in advance..
–im using asp.net and C#