How Can I validate DateExtender Control for date from Today To The Current Year ?

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VB

ASP.NET

I am using gridview control , and in gridview i used a textbox with dateextender cotrol . Now I wanted to validate the textbox for the current year and date must be greater than today.
some code inside gridview

<ItemTemplate>
                                        <asp:TextBox ID="txtdeadline" contentEditable="False" runat="server"
                                Height="28px" Width="57px"></asp:TextBox>
                                        <asp:CalendarExtender ID="txtdeadline_CalendarExtender" runat="server"
                                Enabled="True" TargetControlID="txtdeadline">
                                        </asp:CalendarExtender>


                                    </ItemTemplate>