Ajax calendar Extender Selected date save in variable problem..

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C#

ASP.NET

C#4.0

This is my Html Code...

<asp:UpdatePanel ID="Profits" runat="server" >
  <ContentTemplate>
  <asp:GridView Width="100%" border="1" ID="gwProfit" RowStyle-Wrap="true" runat="server" AutoGenerateColumns="false"
                                CssClass="datatable" AlternatingRowStyle-CssClass="alternaterow" FooterStyle-CssClass="datatableFooter"
                                FooterStyle-VerticalAlign="Bottom" HeaderStyle-CssClass="datatableHeader" RowStyle-VerticalAlign="Bottom"
                                ShowFooter="True" HeaderStyle-Wrap="true" OnRowDataBound="gwProfit_RowDataBound">
                                <Columns>
                                <asp:TemplateField HeaderText="Start Date">
                                        <ItemTemplate>
                   <%--    <div class="input-append">--%>
                       <asp:TextBox class="input_disabled" ID="txtStartDate" runat="server" Width="40px"  Height="22px">
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
                    </asp:TextBox>
                    <button class="btn123" ID="ImgStartDate" runat="server" type="button" style="height:25px;">
                    <%--<i class="icon-calendar"></i>--%>
                    </button>
                    <%--      </div>--%>
                    <ajaxToolkit:CalendarExtender ID="StartDate" runat="server" TargetControlID="txtStartDate"
                    PopupButtonID="ImgStartDate" Format="dd-MMM-yyyy">
                    </ajaxToolkit:CalendarExtender>
                    </ItemTemplate>
                    <FooterTemplate>
            <asp:TextBox class="input_disabled" ID="txtNewStartDate" runat="server" Width="110px"  Height="22px">
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
            </asp:TextBox>
            <button class="btn123" ID="ImgStartDate" runat="server" type="button" style="height:25px;">
            <i class="icon-calendar"></i>
            </button>
            </div>
            <ajaxToolkit:CalendarExtender ID="StartDate" runat="server" TargetControlID="txtNewStartDate"
            PopupButtonID="ImgStartDate" Format="dd-MMM-yyyy">
            </ajaxToolkit:CalendarExtender>
            </FooterTemplate>
           </asp:TemplateField>

And COde behind I am wirting thid code....
GridViewRow row = gwProfit.Rows[indx];
TextBox txtRoomCategory = row.FindControl("txtStartDate") as TextBox;
DateTime dt = Convert.todatetime(txtRoomCategory.Text);


i want to do this selected save in dt but every time txtRoomCategory is Null...
Please Help me.....