diff --git a/wp-includes/feed-rss2-comments.php b/wp-includes/feed-rss2-comments.php
index 09e52410e..ac340b526 100644
--- a/wp-includes/feed-rss2-comments.php
+++ b/wp-includes/feed-rss2-comments.php
@@ -27,7 +27,7 @@ echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>';
 	<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
 	<link><?php (is_single()) ? the_permalink_rss() : bloginfo_rss("url") ?></link>
 	<description><?php bloginfo_rss("description") ?></description>
-	<pubDate><?php echo gmdate('r'); ?></pubDate>
+	<lastBuildDate><?php echo mysql2date('r', get_lastcommentmodified('GMT')); ?></lastBuildDate>
 	<?php the_generator( 'rss2' ); ?>
 	<sy:updatePeriod><?php echo apply_filters( 'rss_update_period', 'hourly' ); ?></sy:updatePeriod>
 	<sy:updateFrequency><?php echo apply_filters( 'rss_update_frequency', '1' ); ?></sy:updateFrequency>