diff --git a/src/wp-includes/feed-rss2-comments.php b/src/wp-includes/feed-rss2-comments.php
index 73d92ce6cc..c8e8d498ea 100644
--- a/src/wp-includes/feed-rss2-comments.php
+++ b/src/wp-includes/feed-rss2-comments.php
@@ -43,7 +43,7 @@ do_action( 'rss_tag_pre', 'rss2-comments' );
 	<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_last_build_date_feed(), false ); ?></lastBuildDate>
+	<lastBuildDate><?php echo mysql2date('r', get_lastcommentmodified('GMT')); ?></lastBuildDate>
 	<sy:updatePeriod><?php
 		/** This filter is documented in wp-includes/feed-rss2.php */
 		echo apply_filters( 'rss_update_period', 'hourly' );
diff --git a/src/wp-includes/feed-rss2.php b/src/wp-includes/feed-rss2.php
index 08802e1e55..c82fc63c56 100644
--- a/src/wp-includes/feed-rss2.php
+++ b/src/wp-includes/feed-rss2.php
@@ -42,7 +42,7 @@ do_action( 'rss_tag_pre', 'rss2' );
 	<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_last_build_date_feed(), false); ?></lastBuildDate>
+	<lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_lastpostmodified('GMT'), false); ?></lastBuildDate>
 	<language><?php bloginfo_rss( 'language' ); ?></language>
 	<sy:updatePeriod><?php
 		$duration = 'hourly';
diff --git a/src/wp-includes/feed.php b/src/wp-includes/feed.php
index ed859c9c47..41c7dac432 100644
--- a/src/wp-includes/feed.php
+++ b/src/wp-includes/feed.php
@@ -87,41 +87,6 @@ function get_default_feed() {
 	return 'rss' == $default_feed ? 'rss2' : $default_feed;
 }
 
-/**
- * Gets 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();
-		$post_times = 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.
  *