+2  A: 

Take a look at http://docs.jquery.com/UI/Datepicker#option-dateFormat

jigfox
A: 

datejs may be useful to you.

Thr4wn
A: 

In addition to the formatting options given by others, you should add using date objects rather than to the string representation of the date object.

I.E.

// add 5 days to today
var myDate=new Date();
myDate.setDate(myDate.getDate()+5);
Joel Potter
A: 

I think you need to detail what jQuery plugin do you use. Is it this one? http://jqueryui.com/demos/datepicker/

If so, then when you cann getDate method, you'll get Date object representing a date. You can easily manipulate it.

Vladimir Grichina
A: 

The date format has nothing to do with how dates are stored; it only affects the way dates are displayed. JavaScript has a native Date object and jQuery UI's Datepicker allows to access such object:

http://jqueryui.com/demos/datepicker/#method-getDate

Once you have a Date object, you can alter it to suit your needs:

https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Date

Finally, you can feed it back into Datepicker:

http://jqueryui.com/demos/datepicker/#method-setDate

Álvaro G. Vicario