Index: wp-includes/version.php
===================================================================
--- wp-includes/version.php	(revision 6728)
+++ wp-includes/version.php	(working copy)
@@ -16,6 +16,6 @@
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 6689;
+$wp_db_version = 6730;
 
 ?>
\ No newline at end of file
Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 6728)
+++ wp-includes/comment.php	(working copy)
@@ -207,10 +207,10 @@
 
 	switch ( strtolower($timezone)) {
 		case 'gmt':
-			$lastcommentmodified = $wpdb->get_var("SELECT comment_date_gmt FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1");
+			$lastcommentmodified = $wpdb->get_var("SELECT comment_date_gmt FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date DESC LIMIT 1");
 			break;
 		case 'blog':
-			$lastcommentmodified = $wpdb->get_var("SELECT comment_date FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1");
+			$lastcommentmodified = $wpdb->get_var("SELECT comment_date FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date DESC LIMIT 1");
 			break;
 		case 'server':
 			$lastcommentmodified = $wpdb->get_var($wpdb->prepare("SELECT DATE_ADD(comment_date_gmt, INTERVAL %s SECOND) FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1", $add_seconds_server));
Index: wp-admin/includes/schema.php
===================================================================
--- wp-admin/includes/schema.php	(revision 6728)
+++ wp-admin/includes/schema.php	(working copy)
@@ -52,7 +52,8 @@
   user_id bigint(20) NOT NULL default '0',
   PRIMARY KEY  (comment_ID),
   KEY comment_approved (comment_approved),
-  KEY comment_post_ID (comment_post_ID)
+  KEY comment_post_ID (comment_post_ID),
+  KEY comment_approved_date (comment_approved,comment_date)
 ) $charset_collate;
 CREATE TABLE $wpdb->links (
   link_id bigint(20) NOT NULL auto_increment,