Datepicker's week calculate error ??

Datepicker's week calculate error ??

   I want to set Friday as the first day of the week. Then i write this code:

  1. $(".date").datepicker({ dateFormat: 'yy-mm-dd', changeMonth: true, changeYear: true, showWeek: true, firstDay: 5 });
When I change date to 2010-01-01(friday), the week displays 53, I think it should be 1.

I am not sure if it is a bug or only my misuse. Thanks for any help!