C# SQL boş string data'nın datetime a nasıl dönüştürülür

DateTime sGirisTarihi = new Datetime();

tring query = "SELECT TARIH    FROM DATABASE_NAME "
                            + "    WHERE SART=@SART "
                            + "    ORDER BY ID DESC";

 using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
             {
                 using (SqlCommand cmd = new SqlCommand(query, conn))
                 {
                     cmd.Parameters.Add("@SART", SqlDbType.Int).Value = 1;
                     conn.Open();
                     using (SqlDataReader rows = cmd.ExecuteReader())
                     {
                         DateTime? dt = String.IsNullOrEmpty(rows["ARACGELISTARIHI"].ToString()) ? (DateTime?)null :                         
                                                       DateTime.Parse(rows["ARACGELISTARIHI"].ToString());
                        sGirisCikisTarihi = Convert.ToDateTime(dt);
                   }
               }
                conn.Close()
           }