in the calander extender have property Format to assign a specific foramted date to textbox try it... Format="dd-MMMM-YY";
<asp:CalendarExtender ID="txt_date_CalendarExtender" runat="server"
Enabled="True" TargetControlID="txt_date" Format="yyyy-MM-dd">
</asp:CalendarExtender>
it show date like 2014-12-05