why datepicker not fill input box if drop down list selected

Question

why datepicker not fill input box if drop down list selected text

 

i have strange problem 

 

when i open page and then select datetimepicker it work and file input box 

 

after that i select data from drop downlist and then 

 

back to datetime picker to change date it not accept any change

 

if i select any new date not affect on input textbox why

 

datetime picker only work if not select any data from drop down list

why