这是一个非常简单的$wpdb->posts 桌子

$last_update = $wpdb->get_var("
  SELECT post_date 
  FROM {$wpdb->posts} 
  WHERE post_type IN(\'post\',\'page\') 
  AND post_status = \'publish\'
  ORDER BY post_date DESC LIMIT 1"
);
var_dump($last_update);

$args = array(
  \'post_type\' => array(\'post\',\'page\'),
  \'posts_per_page\' => 1,
);
$last_updated = new WP_Query($args);
var_dump($last_updated->post->post_date);