use this it Solves your problem
<cc1:toolkitscriptmanager id="ToolkitScriptManager1" runat="server" xmlns:cc1="#unknown">
</cc1:toolkitscriptmanager>
<cc1:maskededitvalidator id="MaskedEditValidator3" runat="server" xmlns:cc1="#unknown">
ControlExtender="MaskedEditExtenderSD2" ControlToValidate="txtStartDate1"
CssClass="RedLabel" Display="Dynamic" EmptyValueBlurredText="*"
ErrorMessage="MaskedEditValidator10" InvalidValueBlurredMessage="Invalid Date"
InvalidValueMessage="Valid Start of Date required" IsValidEmpty="False"
ValidationExpression="^\d{2}/\d{2}/\d{4}$" ValidationGroup="Form">(dd/mm/yyyy)
</cc1:maskededitvalidator>
<cc1:calendarextender id="CalendarExtenderSD2" runat="server" enabled="True" xmlns:cc1="#unknown">
Format="dd/MM/yyyy" PopupButtonID="ImageButtonCalendar1"
TargetControlID="txtStartDate1"></cc1:calendarextender>
<cc1:maskededitextender id="MaskedEditExtenderSD2" runat="server" xmlns:cc1="#unknown">
ClearMaskOnLostFocus="False" Enabled="True" Mask="99/99/9999" MaskType="Date"
TargetControlID="txtStartDate1" CultureAMPMPlaceholder=""
CultureCurrencySymbolPlaceholder="" CultureDateFormat=""
CultureDatePlaceholder="" CultureDecimalPlaceholder=""
CultureThousandsPlaceholder="" CultureTimePlaceholder="" CultureName="pt-BR">
</cc1:maskededitextender>
<asp:textbox id="txtStartDate1" runat="server" xmlns:asp="#unknown"></asp:textbox>
i hope it helps you