If you drag on another icon does it still show the 21st? Mine has always shown the 31st. Remember this is an icon, not a widget, so that could just be the default style.
Try clearing the cache of the calendar app and see if that helps.Prior to upgrading to Jelly Bean it always showed the correct date, and does most of the time. This just happens periodically. Not a big issue.
Try clearing the cache of the calendar app and see if that helps.
go to settings -- apps -- all -- calendar -- clear cache.