Changing date at run time using calendar extender.

Hello everyone,

        I am using image button and ajax calendar extender. If user clicks on image button then calendar pops up ,user can select the dates . After inserting into database if user changes the  date at run time  I need to calculate the difference between the first and second dates and should display it in text box.

       I calculated the difference in the onclick event of image button. But onclick event doesn't fire.