diff --git wp-includes/class-wp.php wp-includes/class-wp.php
index dc77688..7511bcf 100644
--- wp-includes/class-wp.php
+++ wp-includes/class-wp.php
@@ -347,9 +347,9 @@ class WP {
 					)
 				)
 			)
-				$wp_last_modified = mysql2date('D, d M Y H:i:s', get_lastcommentmodified('GMT'), 0).' GMT';
-			else
-				$wp_last_modified = mysql2date('D, d M Y H:i:s', get_lastpostmodified('GMT'), 0).' GMT';
+				$comment_last_modified = strtotime(get_lastcommentmodified('GMT'));
+			$post_last_modified = strtotime(get_lastpostmodified('GMT'));
+			$wp_last_modified = date('D, d M Y H:i:s', isset($comment_last_modified) && $comment_last_modified > $post_last_modified ? $comment_last_modified : $post_last_modified).' GMT';
 			$wp_etag = '"' . md5($wp_last_modified) . '"';
 			$headers['Last-Modified'] = $wp_last_modified;
 			$headers['ETag'] = $wp_etag;