query regarding the Ajax calendaExtender

Question

Dear Friends,
i have a query regarding the Ajax calendaExtender...
in my form there are two date controls Start Date & End Date....i have use calendar extender on them...as well there is Dropdownlist control named PO Number
i have written some code on its SelectedIndex_Change event....my problem is whenever i enter the Date's and select the PO Number from dropdownlist control the date values gets vanished or refreshed.....i also dont want the user to Edit date this so i made the Date's control(Start & End Date) as Read Only

How can i get the Date values even if page gets refreshed on SelectedIndex_Change event
of Dropdownlist control.

Pl reply,
Aamir Khan