session_cache_limiter('');
header("X-Powered-by: MODx - The Greatest Little CMS in the World");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header("Cache-Control: must-revalidate, no-cache, post-check=0, pre-check=0");
header("Pragma: no-cache");
and get this date from DB...