I found this: http://codex.wordpress.org/Displaying_Posts_Using_a_Custom_Select_Query
I have modified the date to retrieve the last 15 days. It’s untested, but should work.
$expireDays="15";
$querystr = "
SELECT $wpdb->posts.*
FROM $wpdb->posts, $wpdb->postmeta
WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id
AND $wpdb->posts.post_status="publish"
AND $wpdb->posts.post_type="post"
AND $wpdb->posts.post_date >= ( CURDATE() - INTERVAL $expireDays DAY )
ORDER BY $wpdb->posts.post_date DESC
";
$pageposts = $wpdb->get_results($querystr, OBJECT);