Date validation format yyyy-MM-dd

     <asp:TextBox ID="txtto" runat="server"></asp:TextBox>// textbox
                      // calender theme
    <asp:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="txtto"      Format="yyyy-MM-dd" CssClass="cal_Theme1" />
                        // validation
 <asp:RegularExpressionValidator ID="RegularExpressionValidatorfatdob" runat="server" ErrorMessage="*" ForeColor="Red" ValidationExpression="^(19|20)\d\d[\- /.](0[1-9]|1[012])[\- /.](0[1-9]|[12][0-9]|3[01])$" ValidationGroup="valsearch" ControlToValidate="txtto"></asp:RegularExpressionValidator>