In wordpress if we want to sort a post by update time but the default is by date. I found if I want to change to sort by update time I should change file ./wp-includes/query.php. Find $q['orderby'] = ‘post_date ‘.$q['order']; and replace by $q['orderby'] = ‘post_modified ‘.$q['order']; (in word press Version 2.3.2 we change line 1125 of query.php)

:)