As @Bluetoft says, the answer is event delegation.

The reason it's not working for you is because your datepicker is being bound to elements that exist at the time the script is run (not to mention, the birthday field has a different id than #datepicker, which is what your script is binding to).

When you dynamically bring in the new form, the datepicker is not bound to the new elements.

So, one method, according to this answer, is to construct your script like so:

$(function() {

    $(body).delegate(#datepicker, focusin, function(){

        $(this).datepicker();

    });

});

Additionally, your form birthday input has an id of #fi_bday, which is another reason it's not being bound. I'd recommend that you assign any inputs where you want the datepicker with a class of datepicker, then change your script to bind the datepicker functionality to '.datepicker' elements:

$(function() {

    $(body).delegate(.datepicker, focusin, function(){

        $(this).datepicker();

    });

});

Finally, if you don't want to utilize the better, generic class method above, you can use two selectors in the selector below. The method below delegates in a manner as to bind the datepicker to the modal window element(s) as well:

$(function() {

    $(body).delegate(#datepicker, #fi_bday, focusin, function(){

        $(this).datepicker();

    });

});