using (con)
        {
            con.Open();

            gv.DataSource =cmd.ExecuteReader();
            gv.DataBind();
                
            con.Close();
        }

<asp:GridView ID="gv" runat="server" AutoGenerateColumns="False">
        <Columns>
            <asp:BoundField DataField="dd_mm_yyyy" DataFormatString="{0:d}" HeaderText="dd_mm_yyyy"   Visible="False"/>
            <asp:BoundField DataField="n_inter" HeaderText="n_inter" />
            <asp:BoundField DataField="interv" HeaderText="interv" />
            <asp:BoundField DataField="vid_en" HeaderText="vid_en" />
            <asp:BoundField DataField="val" HeaderText="val" />
            <asp:BoundField DataField="en_buy" HeaderText="en_buy" />
            <asp:BoundField DataField="en_sell" HeaderText="en_sell" />
            <asp:BoundField DataField="en_pass" HeaderText="en_pass" />
        </Columns>
        </asp:GridView>

 <asp:TextBox ID="tb_date" runat="server" ReadOnly="True" Text=<%# Eval("dd_mm_yyyy") %> ></asp:TextBox></th>
 

tb_date.Text=reader.GetOracleDate(1).Day.ToString()+":"+
                reader.GetOracleDate(1).Month.ToString()+":"+reader.GetOracleDate(1).Year.ToString();

  tb_date.Text = gv.Rows[0].Cells[0].Text;