problem with datepicker

problem with datepicker

hello !
The altFormat option seems to doesn't work
I have this code
$('.datepicker').datepicker({
        showOn: 'button',
        buttonImage: 'images/calendar.gif',
        buttonImageOnly: true,
        numberOfMonths: 2,
        showButtonPanel: true,
        altFormat: 'yy-mm-dd'
    });
and in my input the date format is 'mm-dd-yy'
is that normal ?
I use Firefox 3.5 browser on vista...
thanks !
--