ต้องการคำนวณยอดขายย้อนหลัง 3 เดือน ต้องทำอย่างไรคะ เขียนโดยใช้ c#

 private void get3month()
        {
            string strd, strd1, strd2, strd3, strm, stry, strdt;
            strd1 = "/31/";
            strd2 = "/30/";
            strd3 = "/29/";
            strd = cb_date.EditValue.ToString().Substring(3, 2);
            strm = cb_date.EditValue.ToString().Substring(0, 2);
            stry = cb_date.EditValue.ToString().Substring(6, 4);

            if ()
            {
            }
            strdt = strm + strd + stry;
            txtdate.EditValue = strdt;

              DT = (Convert.ToInt32(StrMount) - Mount);//month now - month select = month show
              if ((DT < 0))//month show < 0
              {
                  DT = 0; //month show = 0
                  DT = (Mount - Convert.ToInt32(StrMount));//month show = month select - month now
                  Str3 = cbdate_str.Text.Substring(6, 4);//year now
                  DT2 = (12 - DT);//12 - montn show = month show2
                  DT3 = (Convert.ToInt32(Str3) - 1);//year now - 1 = year show
                  X = DT2.ToString().Length;
                  if ((X == 1))
                  {
                      Dateback = ("0" + DT2.ToString());
                      Str2 = (Dateback
                                  + (cbdate_str.Text.Substring(2, 4) + DT3.ToString()));
                  }
                  else
                  {
                      Str2 = (DT2.ToString()
                                  + (cbdate_str.Text.Substring(2, 4) + DT3.ToString()));
                  }
                  MessageBox.Show(Str2);
                  dsca = ojsql2.GetDataSet("select month(ddate) dmonth,sum(iquantity) qty,sum(isum )amt from salebillvouch a inner join salebillvouchs b on a.sbvid=b.sbvid where cinvcode='" + txtpd_code.Text + "' and ddate>='" + cbdate_str.EditValue + "' and ddate<='" + Str2 + "' group by month(ddate)", "data");
                  gvFile.DataSource = dsca.Tables[0];

              }
              else
              {
                  Str3 = cbdate_str.Text.Substring(6, 4);
                  DT3 = (Convert.ToInt32(Str3) - 1);
                  X = DT.ToString().Length;
                  if ((X == 1))
                  {
                      Dateback = ("0" + DT.ToString());
                      if ((Dateback == "00"))
                      {
                          Str2 = ("12"
                                      + (cbdate_str.Text.Substring(2, 4) + DT3.ToString()));
                      }
                      else
                      {
                          Str2 = (Dateback
                                      + (cbdate_str.Text.Substring(2, 3) + cbdate_str.Text.Substring(5, 5)));
                      }
                  }
                  else if ((DT.ToString() == "00"))
                  {
                      Str2 = ("12"
                                  + (cbdate_str.Text.Substring(2, 4) + DT3.ToString()));
                  }
                  else
                  {
                      Str2 = (DT.ToString()
                                  + (cbdate_str.Text.Substring(2, 3) + cbdate_str.Text.Substring(5, 5)));
                  }
                  MessageBox.Show(Str2);
                  dsca = ojsql2.GetDataSet("select month(ddate) dmonth,sum(iquantity) qty,sum(isum )amt from salebillvouch a inner join salebillvouchs b on a.sbvid=b.sbvid where cinvcode='" + txtpd_code.Text + "' and ddate>='" + Str2 + "' and ddate<='" + cbdate_str.EditValue + "' group by month(ddate)", "data");
                  gvFile.DataSource = dsca.Tables[0];
              }
        }