I tried adding a CalendarExtender (from the ajaxControlToolkit) within an UpdatePanel control, but everytime I tried triggering the calendar the updatepanel would refresh and i wouldn't be able to choose a date.
Frankly, I don't know why this happens, but I solved it by adding a CalendarExtender somewhere on the page outside the UpdatePanel control. I attached this CalendarExtender to a TextBox with Visible=false so no change is recognized...