boundfield中应该这样设置：

<asp:BoundField HeaderText="发表时间" DataField="PostTime" HtmlEncode=false DataFormatString="{0:yyyy-MM-dd}" >

模板中设置

<asp:TemplateField HeaderText ="日期">
                        <ItemTemplate>
                            <asp:TextBox ID="Date" runat="server" Text='<%# Bind("Date", "{0:d}") %>' BorderStyle="None" 
                            HtmlEncode="True"></asp:TextBox>
                        </ItemTemplate>
                    </asp:TemplateField>