jQuery UI datepicker 'setDate' method not working

I'm having some issues with datepicker in jQuery.

I'm trying to put the today date in a field which is a datepicker.

I've tried with:

$('#fecha_alta').datepicker('setDate', new Date());
console.log($('#fecha_alta').datepicker('getDate'));

And

$('#fecha_alta').datepicker('setDate', '+0');
console.log($('#fecha_alta').datepicker('getDate'));

But always log null and also the input shows empty.

What's wrong?

Copyright Notice：Content Author:「Antonio Laguna」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/7273225/jquery-ui-datepicker-setdate-method-not-working