Hi,
why not you alter the StoredProcedure with additional parameter 'logout_time' and pass the value
string username = Session["uname"].ToString();
string password = Session["pwd"].ToString();
lbl_1.Text = username;
cn.Open();
SqlCommand cmd = new SqlCommand();
cmd.Connection = cn;
cmd.CommandType = CommandType.StoredProcedure;
cmd.CommandText = "SP_TRACK_USE";
cmd.Parameters.AddWithValue("@Username", username);
cmd.Parameters.AddWithValue("@Password", password);
cmd.Parameters.AddWithValue("@logout_time", DateTime.Now());
cmd.Parameters.AddWithValue("@COMMAND", 1);
cmd.ExecuteScalar();
cn.Close();
Response.Redirect("Default.aspx");