Proper way to add a callback to jQuery DatePicker

I wrote a small demo that does this...

I create an object literal that contains the extensions to $.datepicker and then do $.extend on $.datepicker and my object.

You can check it out here: http://jsfiddle.net/NHN4g/4/

Here's the extension itself:

(function($){
    var datepickerExtensions = {
        _oldAdjustDate: $.datepicker._adjustDate,
        _adjustDate: function(id, offset, period) { 
            var target = $(id);
            var inst = this._getInst(target[0]);
            var afterAdjustDate = this._get(inst, 'afterAdjustDate');
            this._oldAdjustDate(id, offset, period);
            if(afterAdjustDate && typeof afterAdjustDate === 'function'){
                afterAdjustDate(id, offset, period);
            }
        }
    }
    $.extend($.datepicker, datepickerExtensions);
})(jQuery);

And the demo:

(html)

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/themes/base/jquery-ui.css" type="text/css" media="all">
<div class="demo">
    <p>Date: <input type="text" id="datepicker"></p>
</div><!-- End demo -->

(javascript)

var changed = false;
$("#datepicker").datepicker({ 
    afterAdjustDate: function(i,o,p){
        if(!changed){
            $('.ui-datepicker-month').css('color', '#f00');
        }
        changed = !changed;
    }
});

There is an onSelect option that you can use to pass a callback function to the datepicker. It sounds like this might be a built-in solution to what you need.

$('.datepicker').datepicker({
  onSelect: function(selectedDate) {
    // custom callback logic here
    alert(selectedDate);
  }
});