I have spent hours on searching on this subject and finally found !! :

a) on a form , drop a dropdown + a datepicker + edit .
b) hook an event in dropdown onchange
c) set edit currencyoptions.enable = true
d) on datepicker set JQMaskoptions set mask to 99/99/9999 and enable := true
run the application , now everytime you change the dropdown selection the edit is cleared

set datepicker mask to enable = false and everything works fine