fix config option

lib/plugins/config/lang/en/lang.php

@@ -171,6 +171,7 @@ $lang['xsendfile']   = 'Use the X-Sendfile header to let the webserver deliver s
 $lang['renderer_xhtml']   = 'Renderer to use for main (xhtml) wiki output';
 $lang['renderer__core']   = '%s (dokuwiki core)';
 $lang['renderer__plugin'] = '%s (plugin)';
+$lang['date_at_format'] = 'Date at format (see PHP\'s <a href="http://www.php.net/manual/datetime.formats.php">datetime.formats</a> information). If empty UNIX timestamp format will be used.';
 
 /* Network Options */
 $lang['dnslookups'] = 'DokuWiki will lookup hostnames for remote IP addresses of users editing pages. If you have a slow or non working DNS server or don\'t want this feature, disable this option';

lib/plugins/config/settings/config.metadata.php

@@ -209,6 +209,7 @@ $meta['broken_iua']  = array('onoff');
 $meta['xsendfile']   = array('multichoice','_choices' => array(0,1,2,3),'_caution' => 'warning');
 $meta['renderer_xhtml'] = array('renderer','_format' => 'xhtml','_choices' => array('xhtml'),'_caution' => 'warning');
 $meta['readdircache'] = array('numeric');
+$meta['date_at_format'] = array('string');
 
 $meta['_network']    = array('fieldset');
 $meta['dnslookups']  = array('onoff');