Many thanks. I've put this shown below and the key name is based on the item also shown below but I've got the following error (in italic)
    DataKeyNames="itm_start_dt"

    <asp:TemplateField HeaderText="Start Date" HeaderStyle-HorizontalAlign="Left" >
    <ItemTemplate>
    <asp:TextBox runat="server" ID="itm_start_dt" HtmlEncode="False" ForeColor="DarkBlue" BackColor="AntiqueWhite" BorderStyle="None"
    ReadOnly="true" Width="65px" Text='<%# Eval("start_dt") %>'/>
    </ItemTemplate>
    <EditItemTemplate>
    <asp:TextBox runat="server" ID="edi_start_dt" HtmlEncode="False"
    Enabled="true" Width="65px" Text='<%# Eval("start_dt") %>' />
    </EditItemTemplate>
    </asp:TemplateField>

Server Error in '/' Application.
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DataBinding: 'System.Data.DataRowView' does not contain a property with the name 'chk_start_dt2'.