Date substraction

str2 = "select FINAL_ORD_DATE from cases where grf_id='" + grf_id + "' and cc_year='" + year + "' order by cc_no asc";
            _c1.Execute2(str2, ref _dtt);
            str2 = string.Empty;

            str = "select REGN_DATE from cases grf_id='" + grf_id + "' and cc_year='" + year + "' order by cc_no asc";
            _c1.Execute2(str, ref _dtt);
            str = string.Empty;
            DateTime dt1 = DateTime.Parse(str2);

            DateTime dt2 = DateTime.Parse(str1);
            TimeSpan span = dt2.Subtract(dt1);
            lblorderdate.Text = str2;
            span = Convert.ToDouble(lblorderdate.Text);


Kindly check this code and rectify it