[ticket/13044] Correct Expires headers

PHPBB3-13044

phpBB/includes/functions_acp.php

@@ -112,7 +112,7 @@ function adm_page_header($page_title)
 		// application/xhtml+xml not used because of IE
 		'Content-type' => 'text/html; charset=UTF-8',
 		'Cache-Control' => 'private, no-cache="set-cookie"',
-		'Expires' => '0',
+		'Expires' => gmdate('D, d M Y H:i:s \G\M\T', time()),
 		'Pragma' => 'no-cache',
 	);