How do i select data from a dynamic date range

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C#

SQL

here is my code; i want to search from a dynamic date range where start range is selected from my DB and end range will be the date after 3 month of that starting range.

string sqlquery6 = "SELECT * FROM Bill_Details where PCID= '"+PCNo+"' order by Bill_Date";
            SqlDataAdapter adp6 = new SqlDataAdapter(sqlquery6, conn);
            DataTable tbl6 = new DataTable();
            adp6.Fill(tbl6);
            if (tbl6.Rows.Count > 0)
            {
                DateTime startrange = Convert.ToDateTime(tbl6.Rows[0][2].ToString());
                DateTime endrange = Convert.ToDateTime(startrange + 90);
                string sqlquery3 = "SELECT * FROM Bill_Details WHERE Bill_Date BETWEEN '" + startrange + "' AND '" + endrange + "' and PCID= '" + PCNo + "'";
                SqlDataAdapter adp3 = new SqlDataAdapter(sqlquery3, conn);
                DataTable tbl3 = new DataTable();
                adp3.Fill(tbl3);
                if (tbl3.Rows.Count > 0)
                {
                    int totbill = 0;
                    for (int i = 0; i < tbl3.Rows.Count; i++)
                    {
                        totbill = totbill + Convert.ToInt32(tbl3.Rows[i][0].ToString());
                    }

                    txttot.Text = totbill.ToString();
                }
                else
                {
                    txttot.Text = "0"; ;
                }
            }