Problem with Mask Edit Extendar

Hi All,
I have write this code for date Validation

<asp:TextBox ID="txtDate" runat="server" onchange="return ValidateDate(this);" ></asp:TextBox>

<asp:MaskedEditExtender ID="txtDate_MaskedEditExtender" runat="server"
TargetControlID="txtDate" Mask="99/99/9999" MaskType="DateTime" >
</asp:MaskedEditExtender>

<asp:CalendarExtender ID="txtDate_CalendarExtender" runat="server"
Enabled="True" TargetControlID="txtDate" PopupButtonID="imgDate" Format="dd/MM/yyyy">
</asp:CalendarExtender>

<asp:Image ID="imgDate" runat="server" Height="21px"
ImageUrl="~/Image/Calendar_Icon.jpg" Width="26px" />

where TextBox onchange event i am calling the javascript validation function, if i remove the maskedit extendatr it works properly but with maskedit javascript function not call.

what is the reason ?
Thanks