Sorry if this was already fixed, I didn't found any hints about this on the forums. Anyway:
I noticed that in the DateField if you enter a valid date in the text field and click on the dropdown the calendar popup always stays at the same date wheras if you pick a date in the calendar popup the text field is updated.
I think it would be intuitive to work in both ways i.e. text field changes should be reflected in the calendar popup.