我发现了这个：http://codex.wordpress.org/Displaying_Posts_Using_a_Custom_Select_Query
我已修改日期以检索过去15天。这是未经测试的，但应该有效。

$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);

设置过期日期可能比设置期限更容易，然后可以对过期日期大于今天的帖子进行元查询：

$args = array(
            \'posts_per_page\' => -1,
            \'meta_key\' => \'expiration_date\',
            \'meta_value\' => date(\'Y-m-d\'),
            \'meta_compare\' => \'>=\',
            \'orderby\' => \'meta_value\',
            \'order\' => \'ASC\'            
);
$posts = new WP_Query($args);