how to set Years in ajax CalendarExtender in asp.net

<script type="text/javascript" >
    function ShowYears(sender, args ) {
        sender._switchMode("years", true);  
    }

</script>

DOB::
 <asp:TextBox ID="txt_DOB" runat="server"></asp:TextBox>
    <asp:CalendarExtender ID="txt_DOB_CalendarExtender" runat="server" OnClientShown="ShowYears"     
        Enabled="True" TargetControlID="txt_DOB" Format="dd/MM/yyyy">
    </asp:CalendarExtender>