CalendarExtender and validation

Hi:

I try to validate the date once the user choose the calendar they want. However, 

when the value set to the text box, the validate function not invoking immediately.

If that possible to cover this problem, 

the code snippet is as following:

 <asp:TextBox ID="textbox1" runat="server" Font-Size="Large" onchange="validate();"></asp:TextBox>
<cc1:CalendarExtender ID="CalendarExtender1" runat="server" Enabled="True" Format="yyyy/MM/dd" PopupButtonID="Button1" TargetControlID="textbox1"> </cc1:CalendarExtender>
           

Thanks a lot.