display and count posts from last 7 days

Hi, I have the below code which is displaying and counting posts from certain categories from the current month only. I want to do the same thing but for a week instead of month. There doesn’t seem to be an easy solution. is there something like “monthnum” that can be used for “week”? Any help would be great.

$month = date('m');

$posts = query_posts(
 array( 'category__and' => array(6,15),
 'monthnum' => $month,

));

$ricpos = count($posts);