Hi friend,
Use this below code , if u r using doing this by AJAX...
Please Note dd/MM/yyyy(MM should be in Capital)
<p>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="btn" runat="server" Text="..." />
</p>
<p>
</p>
<asp:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="TextBox1"
PopupPosition="BottomRight" Format="dd/MM/yyyy " Animated="true" PopupButtonID="btn"
CssClass=" MyCalender" >
</asp:CalendarExtender>