update panel postback problem

inside upadate panel

i placed one text box and oneimage and one dropdownlist(it contains some list) ,when i click on image one pop up calender widow appear .

i wrote javascript for calender window.

i run my apllication sucessfully.it working fine

but next time i selected dropdownlist item ,and change the date in calender window the date was not changed (first time date changed ,second time not changed )

and all controls placed in outside update panel everthing is working fine

in update panel its not working plz help me