Calendar extender - notify of text changed/date selected?

I have a calendar user control.

This has the ability to get/set a textbox control, which has the calendar extender control applied

The user control also disabled input from the user to avoid invalid input/text. This is using the readonly attribute and the readonly property on the textbox. This works great.

I now need to go a step further - I want to know, via postback, if the selected date has changed.

how can I do this? It should be a full postback, so I can then raise an event to notify the parent that the selected changed has occured.

I also need to state that this 1 ASP.NET page has 3 or 4 user controls, all having a calendar control. So I only want the "designated" one to have this type of functionality.

Thanks