Index: wp-includes/feed-rss2-comments.php
===================================================================
--- wp-includes/feed-rss2-comments.php	(revision 32269)
+++ wp-includes/feed-rss2-comments.php	(working copy)
@@ -43,7 +43,7 @@
 	<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>
-	<lastBuildDate><?php echo mysql2date('r', get_lastcommentmodified('GMT')); ?></lastBuildDate>
+	<lastBuildDate><?php echo mysql2date('r', get_last_build_date_feed(), false ); ?></lastBuildDate>
 	<sy:updatePeriod><?php
 		/** This filter is documented in wp-includes/feed-rss2.php */
 		echo apply_filters( 'rss_update_period', 'hourly' );
Index: wp-includes/feed-rss2.php
===================================================================
--- wp-includes/feed-rss2.php	(revision 32269)
+++ wp-includes/feed-rss2.php	(working copy)
@@ -42,7 +42,7 @@
 	<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
 	<link><?php bloginfo_rss('url') ?></link>
 	<description><?php bloginfo_rss("description") ?></description>
-	<lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_lastpostmodified('GMT'), false); ?></lastBuildDate>
+	<lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_last_build_date_feed(), false); ?></lastBuildDate>
 	<language><?php bloginfo_rss( 'language' ); ?></language>
 	<sy:updatePeriod><?php
 		$duration = 'hourly';
Index: wp-includes/feed.php
===================================================================
--- wp-includes/feed.php	(revision 32269)
+++ wp-includes/feed.php	(working copy)
@@ -88,6 +88,40 @@
 }

 /**
+ * Get the timestamp of the most recently modified post from WP_Query
+ *
+ * If viewing a comment feed, the date of the most recently modified
+ * comment will be returned.
+ *
+ * @since 4.3.0
+ *
+ * @return string Date ('Y-m-d H:i:s' for use with mysql2date() )
+ */
+function get_last_build_date_feed() {
+	global $wp_query, $wpdb;
+
+	if ( $wp_query->have_posts() ) {
+		$post_ids = array();
+		foreach( $wp_query->posts as $post ) {
+			$post_ids[] = $post->ID;
+			$post_times[] = $post->post_modified_gmt;
+		}
+		$postids = implode( "','", $post_ids );
+		$max_post_time = max( $post_times );
+
+		if( $wp_query->is_comment_feed() ) {
+			$max_comment_time = $wpdb->get_var( $wpdb->prepare( "SELECT MAX(comment_date_gmt) FROM $wpdb->comments WHERE comment_post_ID IN ('%s') AND comment_approved = '1'", $postids ) );
+
+			return max( $max_post_time, $max_comment_time );
+		}
+		return $max_post_time;
+	}
+
+	// Fallback to last time any post was modified or published.
+	return get_lastpostmodified( 'GMT' );
+}
+
+/**
  * Retrieve the blog title for the feed title.
  *
  * @since 2.2.0