diff --git src/wp-includes/feed-atom-comments.php src/wp-includes/feed-atom-comments.php
index 64e26aa2fd..fe92ed71fa 100644
--- src/wp-includes/feed-atom-comments.php
+++ src/wp-includes/feed-atom-comments.php
@@ -45,7 +45,7 @@ do_action( 'rss_tag_pre', 'atom-comments' );
 
 	<updated>
 	<?php
-		$date = get_lastcommentmodified( 'GMT' );
+		$date = get_last_build_date();
 		echo $date ? mysql2date( 'Y-m-d\TH:i:s\Z', $date, false ) : date( 'Y-m-d\TH:i:s\Z' );
 	?>
 	</updated>
diff --git src/wp-includes/feed-atom.php src/wp-includes/feed-atom.php
index 73cfda1e5b..a30ae3e3b1 100644
--- src/wp-includes/feed-atom.php
+++ src/wp-includes/feed-atom.php
@@ -32,7 +32,7 @@ do_action( 'rss_tag_pre', 'atom' );
 
 	<updated>
 	<?php
-		$date = get_lastpostmodified( 'GMT' );
+		$date = get_last_build_date();
 		echo $date ? mysql2date( 'Y-m-d\TH:i:s\Z', $date, false ) : date( 'Y-m-d\TH:i:s\Z' );
 	?>
 	</updated>
diff --git src/wp-includes/feed-rdf.php src/wp-includes/feed-rdf.php
index 4fa4147431..708893245f 100644
--- src/wp-includes/feed-rdf.php
+++ src/wp-includes/feed-rdf.php
@@ -35,7 +35,7 @@ do_action( 'rss_tag_pre', 'rdf' );
 	<description><?php bloginfo_rss( 'description' ); ?></description>
 	<dc:date>
 	<?php
-		$date = get_lastpostmodified( 'GMT' );
+		$date = get_last_build_date();
 		echo $date ? mysql2date( 'Y-m-d\TH:i:s\Z', $date ) : date( 'Y-m-d\TH:i:s\Z' );
 	?>
 	</dc:date>
diff --git src/wp-includes/feed-rss.php src/wp-includes/feed-rss.php
index c776c77b56..06d5f66a60 100644
--- src/wp-includes/feed-rss.php
+++ src/wp-includes/feed-rss.php
@@ -16,7 +16,7 @@ echo '<?xml version="1.0" encoding="' . get_option( 'blog_charset' ) . '"?' . '>
 	<description><?php bloginfo_rss( 'description' ); ?></description>
 	<lastBuildDate>
 	<?php
-		$date = get_lastpostmodified( 'GMT' );
+		$date = get_last_build_date();
 		echo $date ? mysql2date( 'D, d M Y H:i:s +0000', $date ) : date( 'D, d M Y H:i:s +0000' );
 	?>
 	</lastBuildDate>
diff --git src/wp-includes/feed-rss2-comments.php src/wp-includes/feed-rss2-comments.php
index b26c913408..c736bebccd 100644
--- src/wp-includes/feed-rss2-comments.php
+++ src/wp-includes/feed-rss2-comments.php
@@ -51,7 +51,7 @@ do_action( 'rss_tag_pre', 'rss2-comments' );
 	<description><?php bloginfo_rss( 'description' ); ?></description>
 	<lastBuildDate>
 	<?php
-		$date = get_lastcommentmodified( 'GMT' );
+		$date = get_last_build_date();
 		echo $date ? mysql2date( 'r', $date, false ) : date( 'r' );
 	?>
 	</lastBuildDate>
diff --git src/wp-includes/feed-rss2.php src/wp-includes/feed-rss2.php
index 6a99165ac7..d7ddccabe0 100644
--- src/wp-includes/feed-rss2.php
+++ src/wp-includes/feed-rss2.php
@@ -44,7 +44,7 @@ do_action( 'rss_tag_pre', 'rss2' );
 	<description><?php bloginfo_rss( 'description' ); ?></description>
 	<lastBuildDate>
 	<?php
-		$date = get_lastpostmodified( 'GMT' );
+		$date = get_last_build_date();
 		echo $date ? mysql2date( 'r', $date, false ) : date( 'r' );
 	?>
 	</lastBuildDate>
diff --git src/wp-includes/feed.php src/wp-includes/feed.php
index c8f5fd5d3d..f8662b3055 100644
--- src/wp-includes/feed.php
+++ src/wp-includes/feed.php
@@ -637,6 +637,51 @@ function self_link() {
 	echo esc_url( apply_filters( 'self_link', set_url_scheme( 'http://' . $host['host'] . wp_unslash( $_SERVER['REQUEST_URI'] ) ) ) );
 }
 
+/*
+* 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.
+*
+* @global WP_Query  $wp_query The global WP_Query object.
+*
+* @since 5.2.0
+*
+* @return string The timestamp.
+*/
+function get_last_build_date() {
+	global $wp_query;
+
+	if ( empty( $wp_query ) || ! $wp_query->have_posts() ) {
+		// Fallback to last time any post was modified or published.
+		return get_lastpostmodified( 'GMT' );
+	}
+
+	// Extract the post modified times from the posts.
+	$modified_times = wp_list_pluck( $wp_query->posts, 'post_modified_gmt' );
+
+	// If this is a comment feed, check those objects too.
+	if ( $wp_query->is_comment_feed() && $wp_query->comment_count ) {
+		// Extract the comment modified times from the comments.
+		$comment_times = wp_list_pluck( $wp_query->comments, 'comment_date_gmt' );
+
+		// Add the comment times to the post times for comparison.
+		$modified_times = array_merge( $modified_times, $comment_times );
+	}
+
+	// Determine the maximum modified time.
+	$max_modified_time = max( $modified_times );
+
+	/**
+	 * Filters the date the last post or comment in the query was modified.
+	 *
+	 * @since 5.2.0
+	 *
+	 * @param string $max_modified_times Date the last post or comment was modified in the query.
+	 */
+	return apply_filters( 'get_last_build_date', $max_modified_time );
+}
+
 /**
  * Return the content type for specified feed type.
  *
diff --git tests/phpunit/tests/feed/rss2.php tests/phpunit/tests/feed/rss2.php
index 79be2fcc64..c8c3f32c2f 100644
--- tests/phpunit/tests/feed/rss2.php
+++ tests/phpunit/tests/feed/rss2.php
@@ -36,20 +36,23 @@ class Tests_Feeds_RSS2 extends WP_UnitTestCase {
 		);
 
 		// Set a predictable time for testing date archives.
-		self::$post_date = '2003-05-27 10:07:53';
+		self::$post_date = strtotime( '2003-05-27 10:07:53' );
 
 		$count = get_option( 'posts_per_rss' ) + 1;
 
+		self::$posts = array();
 		// Create a few posts
-		self::$posts = $factory->post->create_many(
-			$count,
-			array(
-				'post_author'  => self::$user_id,
-				'post_date'    => self::$post_date,
-				'post_content' => 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec velit massa, ultrices eu est suscipit, mattis posuere est. Donec vitae purus lacus. Cras vitae odio odio.',
-				'post_excerpt' => 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
-			)
-		);
+		for ( $i = 1; $i <= $count; $i ++ ) {
+			self::$posts[] = $factory->post->create(
+				array(
+					'post_author'  => self::$user_id,
+					// Separate post dates 5 seconds apart.
+					'post_date'    => gmdate( 'Y-m-d H:i:s', self::$post_date + ( 5 * $i ) ),
+					'post_content' => 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec velit massa, ultrices eu est suscipit, mattis posuere est. Donec vitae purus lacus. Cras vitae odio odio.',
+					'post_excerpt' => 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
+				)
+			);
+		}
 
 		// Assign a category to those posts
 		foreach ( self::$posts as $post ) {
@@ -396,6 +399,7 @@ class Tests_Feeds_RSS2 extends WP_UnitTestCase {
 		// Queries performed on valid feed endpoints should contain posts.
 		$this->assertTrue( have_posts() );
 
+
 		// Check to see if we have the expected XML output from the feed template.
 		$feed = $this->do_rss2();
 
@@ -463,4 +467,31 @@ class Tests_Feeds_RSS2 extends WP_UnitTestCase {
 		// There should only be one <rss> child element.
 		$this->assertEquals( 1, count( $rss ) );
 	}
+
+	/**
+	 * Test <rss> element has correct last build date.
+	 *
+	 * @ticket 4575
+	 *
+	 * @dataProvider data_test_get_last_build_date
+	 */
+	public function test_get_last_build_date( $url, $element ) {
+		$this->go_to( $url );
+		$feed = $this->do_rss2();
+		$xml  = xml_to_array( $feed );
+
+		// Get the <rss> child element of <xml>.
+		$rss             = xml_find( $xml, $element );
+		$last_build_date = $rss[0]['child'][0]['child'][4]['content'];
+		$this->assertEquals( strtotime( get_last_build_date() ), strtotime( $last_build_date ) );
+	}
+
+
+	public function data_test_get_last_build_date() {
+		return array(
+			array( '/?feed=rss2', 'rss' ),
+			array( '/?feed=commentsrss2', 'rss' ),
+		);
+
+	}
 }