Formatting out of a jQuery UI datepicker

I'm having a problem trying to format the output on the jQuery UI datepicker.

I want the dateformat to be the 'ISO 8601' format, like explained here:

http://jqueryui.com/demos/datepicker/#date-formats

This is what my code looks like:

$.datepicker.setDefaults($.datepicker.regional['nl']);
$('.datepicker').datepicker('option', {dateFormat: 'yy-mm-dd' });