Avoiding full postback when using OnClientDateSelectionChanged on a control

Avoiding full postback when using OnClientDateSelectionChanged on a control

I am using ASP.NET c# 4.0
I am using AJAX.
I am using ajax calendar extender which after selecting a date fills a
text box with a chosen date. I wanted to execute some server side code
when the date changes so I put this in :
OnClientDateSelectionChanged="function
ddd(){__doPostBack('DateTextBox','') ;}" into the extender control.
Unfortunately, the whole page gets posted back. If i put this control in
an Update control it tells me that it is already in an update control. If
I include the textbox within the update panel i get 2 of these text boxes
on my web page after selecting a date. Does anyone know how to inform the
server that the date selected from this ajax control has changed without a
full post back please?
thanks