diff --git a/src/wp-includes/class-wp.php b/src/wp-includes/class-wp.php
index cac538d6cd..c62d79082b 100644
--- a/src/wp-includes/class-wp.php
+++ b/src/wp-includes/class-wp.php
@@ -422,22 +422,30 @@ class WP {
 			}
 			$headers['Content-Type'] = feed_content_type( $type ) . '; charset=' . get_option( 'blog_charset' );
 
-			// We're showing a feed, so WP is indeed the only thing that last changed
-			if ( !empty($this->query_vars['withcomments'])
-				|| false !== strpos( $this->query_vars['feed'], 'comments-' )
-				|| ( empty($this->query_vars['withoutcomments'])
-					&& ( !empty($this->query_vars['p'])
-						|| !empty($this->query_vars['name'])
-						|| !empty($this->query_vars['page_id'])
-						|| !empty($this->query_vars['pagename'])
-						|| !empty($this->query_vars['attachment'])
-						|| !empty($this->query_vars['attachment_id'])
-					)
-				)
-			)
-				$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';
+			// We're showing a feed, so WP is indeed the only thing that last changed.
+			if ( ! empty( $this->query_vars['withcomments'] )
+			     || false !== strpos( $this->query_vars['feed'], 'comments-' )
+			     || ( empty( $this->query_vars['withoutcomments'] )
+			          && ( ! empty( $this->query_vars['p'] )
+			               || ! empty( $this->query_vars['name'] )
+			               || ! empty( $this->query_vars['page_id'] )
+			               || ! empty( $this->query_vars['pagename'] )
+			               || ! empty( $this->query_vars['attachment'] )
+			               || ! empty( $this->query_vars['attachment_id'] )
+			          )
+			     )
+			) {
+				$wp_last_modified = mysql2date( 'D, d M Y H:i:s', get_lastcommentmodified( 'GMT' ), false );
+			} else {
+				$wp_last_modified = mysql2date( 'D, d M Y H:i:s', get_lastpostmodified( 'GMT' ), false );
+			}
+
+			if ( ! $wp_last_modified ) {
+				$wp_last_modified = date( 'D, d M Y H:i:s' );
+			}
+
+			$wp_last_modified .= ' GMT';
+
 			$wp_etag = '"' . md5($wp_last_modified) . '"';
 			$headers['Last-Modified'] = $wp_last_modified;
 			$headers['ETag'] = $wp_etag;
diff --git a/src/wp-includes/comment.php b/src/wp-includes/comment.php
index 49ddda8b6d..7da516266f 100644
--- a/src/wp-includes/comment.php
+++ b/src/wp-includes/comment.php
@@ -293,38 +293,46 @@ function get_default_comment_status( $post_type = 'post', $comment_type = 'comme
  * The date the last comment was modified.
  *
  * @since 1.5.0
+ * @since 4.7.0 Replaced caching the modified date in a local static variable
+ *              with the Object Cache API.
  *
  * @global wpdb $wpdb WordPress database abstraction object.
- * @staticvar array $cache_lastcommentmodified
  *
- * @param string $timezone Which timezone to use in reference to 'gmt', 'blog',
- *		or 'server' locations.
- * @return string Last comment modified date.
+ * @param string $timezone Which timezone to use in reference to 'gmt', 'blog', or 'server' locations.
+ * @return string|false Last comment modified date on success, false on failure.
  */
-function get_lastcommentmodified($timezone = 'server') {
+function get_lastcommentmodified( $timezone = 'server' ) {
 	global $wpdb;
-	static $cache_lastcommentmodified = array();
 
-	if ( isset($cache_lastcommentmodified[$timezone]) )
-		return $cache_lastcommentmodified[$timezone];
+	$timezone = strtolower( $timezone );
+	$key = "lastcommentmodified:$timezone";
 
-	$add_seconds_server = date('Z');
+	$comment_modified_date = wp_cache_get( $key, 'timeinfo' );
+	if ( false !== $comment_modified_date ) {
+		return $comment_modified_date;
+	}
 
-	switch ( strtolower($timezone)) {
+	switch ( $timezone ) {
 		case 'gmt':
-			$lastcommentmodified = $wpdb->get_var("SELECT comment_date_gmt FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1");
+			$comment_modified_date = $wpdb->get_var( "SELECT comment_date_gmt FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1" );
 			break;
 		case 'blog':
-			$lastcommentmodified = $wpdb->get_var("SELECT comment_date FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1");
+			$comment_modified_date = $wpdb->get_var( "SELECT comment_date FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1" );
 			break;
 		case 'server':
-			$lastcommentmodified = $wpdb->get_var($wpdb->prepare("SELECT DATE_ADD(comment_date_gmt, INTERVAL %s SECOND) FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1", $add_seconds_server));
+			$add_seconds_server = date( 'Z' );
+
+			$comment_modified_date = $wpdb->get_var( $wpdb->prepare( "SELECT DATE_ADD(comment_date_gmt, INTERVAL %s SECOND) FROM $wpdb->comments WHERE comment_approved = '1' ORDER BY comment_date_gmt DESC LIMIT 1", $add_seconds_server ) );
 			break;
 	}
 
-	$cache_lastcommentmodified[$timezone] = $lastcommentmodified;
+	if ( $comment_modified_date ) {
+		wp_cache_set( $key, $comment_modified_date, 'timeinfo' );
 
-	return $lastcommentmodified;
+		return $comment_modified_date;
+	}
+
+	return false;
 }
 
 /**
@@ -1572,6 +1580,26 @@ function wp_transition_comment_status($new_status, $old_status, $comment) {
 	do_action( "comment_{$new_status}_{$comment->comment_type}", $comment->comment_ID, $comment );
 }
 
+/**
+ * Clear the lastcommentmodified cached value when a comment status is changed.
+ *
+ * Deletes the lastcommentmodified cache key when a comment enters or leaves
+ * 'approved' status.
+ *
+ * @since 4.7.0
+ * @access private
+ *
+ * @param string $new_status The new comment status.
+ * @param string $old_status The old comment status.
+ */
+function _clear_modified_cache_on_transition_comment_status( $new_status, $old_status ) {
+	if ( 'approved' === $new_status || 'approved' === $old_status ) {
+		foreach ( array( 'server', 'gmt', 'blog' ) as $timezone ) {
+			wp_cache_delete( "lastcommentmodified:$timezone", 'timeinfo' );
+		}
+	}
+}
+
 /**
  * Get current commenter's name, email, and URL.
  *
@@ -1681,6 +1709,10 @@ function wp_insert_comment( $commentdata ) {
 
 	if ( $comment_approved == 1 ) {
 		wp_update_comment_count( $comment_post_ID );
+
+		foreach ( array( 'server', 'gmt', 'blog' ) as $timezone ) {
+			wp_cache_delete( "lastcommentmodified:$timezone", 'timeinfo' );
+		}
 	}
 
 	clean_comment_cache( $id );
diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index 71255c3497..323e55e840 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -214,6 +214,8 @@ add_filter( 'pingback_ping_source_uri', 'pingback_ping_source_uri'            );
 add_filter( 'xmlrpc_pingback_error',    'xmlrpc_pingback_error'               );
 add_filter( 'title_save_pre',           'trim'                                );
 
+add_action( 'transition_comment_status', '_clear_modified_cache_on_transition_comment_status', 10, 2 );
+
 add_filter( 'http_request_host_is_external',    'allowed_http_request_hosts', 10, 2 );
 
 // REST API filters.
diff --git a/src/wp-includes/feed-atom-comments.php b/src/wp-includes/feed-atom-comments.php
index e5c7089164..5b494a2992 100644
--- a/src/wp-includes/feed-atom-comments.php
+++ b/src/wp-includes/feed-atom-comments.php
@@ -37,7 +37,10 @@ do_action( 'rss_tag_pre', 'atom-comments' );
 	?></title>
 	<subtitle type="text"><?php bloginfo_rss('description'); ?></subtitle>
 
-	<updated><?php echo mysql2date('Y-m-d\TH:i:s\Z', get_lastcommentmodified('GMT'), false); ?></updated>
+	<updated><?php
+		$date = get_lastcommentmodified( 'GMT' );
+		echo $date ? mysql2date( 'Y-m-d\TH:i:s\Z', $date ) : date( 'Y-m-d\TH:i:s\Z' );
+	?></updated>
 
 <?php if ( is_singular() ) { ?>
 	<link rel="alternate" type="<?php bloginfo_rss('html_type'); ?>" href="<?php comments_link_feed(); ?>" />
diff --git a/src/wp-includes/feed-atom.php b/src/wp-includes/feed-atom.php
index a4a62f74b9..09cb7c016e 100644
--- a/src/wp-includes/feed-atom.php
+++ b/src/wp-includes/feed-atom.php
@@ -30,7 +30,10 @@ do_action( 'rss_tag_pre', 'atom' );
 	<title type="text"><?php wp_title_rss(); ?></title>
 	<subtitle type="text"><?php bloginfo_rss("description") ?></subtitle>
 
-	<updated><?php echo mysql2date('Y-m-d\TH:i:s\Z', get_lastpostmodified('GMT'), false); ?></updated>
+	<updated><?php
+		$date = get_lastpostmodified( 'GMT' );
+		echo $date ? mysql2date( 'Y-m-d\TH:i:s\Z', $date ) : date( 'Y-m-d\TH:i:s\Z' );
+	?></updated>
 
 	<link rel="alternate" type="<?php bloginfo_rss('html_type'); ?>" href="<?php bloginfo_rss('url') ?>" />
 	<id><?php bloginfo('atom_url'); ?></id>
diff --git a/src/wp-includes/feed-rdf.php b/src/wp-includes/feed-rdf.php
index 8302881500..b748400032 100644
--- a/src/wp-includes/feed-rdf.php
+++ b/src/wp-includes/feed-rdf.php
@@ -33,7 +33,10 @@ do_action( 'rss_tag_pre', 'rdf' );
 	<title><?php wp_title_rss(); ?></title>
 	<link><?php bloginfo_rss('url') ?></link>
 	<description><?php bloginfo_rss('description') ?></description>
-	<dc:date><?php echo mysql2date('Y-m-d\TH:i:s\Z', get_lastpostmodified('GMT'), false); ?></dc:date>
+	<dc:date><?php
+		$date = get_lastpostmodified( 'GMT' );
+		echo $date ? mysql2date( 'Y-m-d\TH:i:s\Z', $date ) : date( 'Y-m-d\TH:i:s\Z' );
+	?></dc:date>
 	<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-rss.php b/src/wp-includes/feed-rss.php
index df231968b7..39202b83b2 100644
--- a/src/wp-includes/feed-rss.php
+++ b/src/wp-includes/feed-rss.php
@@ -14,7 +14,10 @@ echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>'; ?>
 	<title><?php wp_title_rss(); ?></title>
 	<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
+		$date = get_lastpostmodified( 'GMT' );
+		echo $date ? mysql2date( 'D, d M Y H:i:s +0000', $date ) : date( 'D, d M Y H:i:s +0000' );
+	?></lastBuildDate>
 	<docs>http://backend.userland.com/rss092</docs>
 	<language><?php bloginfo_rss( 'language' ); ?></language>
 
diff --git a/src/wp-includes/feed-rss2-comments.php b/src/wp-includes/feed-rss2-comments.php
index f05bb8f71a..af329c5b63 100644
--- a/src/wp-includes/feed-rss2-comments.php
+++ b/src/wp-includes/feed-rss2-comments.php
@@ -43,7 +43,10 @@ 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_lastcommentmodified('GMT')); ?></lastBuildDate>
+	<lastBuildDate><?php
+		$date = get_lastcommentmodified( 'GMT' );
+		echo $date ? mysql2date( 'r', $date ) : date( 'r' );
+	?></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 b5f3d781b2..a7b9ac7ec1 100644
--- a/src/wp-includes/feed-rss2.php
+++ b/src/wp-includes/feed-rss2.php
@@ -42,7 +42,10 @@ 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_lastpostmodified('GMT'), false); ?></lastBuildDate>
+	<lastBuildDate><?php
+		$date = get_lastpostmodified( 'GMT' );
+		echo $date ? mysql2date( 'D, d M Y H:i:s +0000', $date ) : date( 'D, d M Y H:i:s +0000' );
+	?></lastBuildDate>
 	<language><?php bloginfo_rss( 'language' ); ?></language>
 	<sy:updatePeriod><?php
 		$duration = 'hourly';
diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index cd6233fee8..cfb941f8c8 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -5612,35 +5612,38 @@ function _get_last_post_time( $timezone, $field, $post_type = 'any' ) {
 	}
 
 	$date = wp_cache_get( $key, 'timeinfo' );
-
-	if ( ! $date ) {
-		if ( 'any' === $post_type ) {
-			$post_types = get_post_types( array( 'public' => true ) );
-			array_walk( $post_types, array( $wpdb, 'escape_by_ref' ) );
-			$post_types = "'" . implode( "', '", $post_types ) . "'";
-		} else {
-			$post_types = "'" . sanitize_key( $post_type ) . "'";
-		}
-
-		switch ( $timezone ) {
-			case 'gmt':
-				$date = $wpdb->get_var("SELECT post_{$field}_gmt FROM $wpdb->posts WHERE post_status = 'publish' AND post_type IN ({$post_types}) ORDER BY post_{$field}_gmt DESC LIMIT 1");
-				break;
-			case 'blog':
-				$date = $wpdb->get_var("SELECT post_{$field} FROM $wpdb->posts WHERE post_status = 'publish' AND post_type IN ({$post_types}) ORDER BY post_{$field}_gmt DESC LIMIT 1");
-				break;
-			case 'server':
-				$add_seconds_server = date( 'Z' );
-				$date = $wpdb->get_var("SELECT DATE_ADD(post_{$field}_gmt, INTERVAL '$add_seconds_server' SECOND) FROM $wpdb->posts WHERE post_status = 'publish' AND post_type IN ({$post_types}) ORDER BY post_{$field}_gmt DESC LIMIT 1");
-				break;
-		}
-
-		if ( $date ) {
-			wp_cache_set( $key, $date, 'timeinfo' );
-		}
+	if ( false !== $date ) {
+		return $date;
 	}
 
-	return $date;
+	if ( 'any' === $post_type ) {
+		$post_types = get_post_types( array( 'public' => true ) );
+		array_walk( $post_types, array( $wpdb, 'escape_by_ref' ) );
+		$post_types = "'" . implode( "', '", $post_types ) . "'";
+	} else {
+		$post_types = "'" . sanitize_key( $post_type ) . "'";
+	}
+
+	switch ( $timezone ) {
+		case 'gmt':
+			$date = $wpdb->get_var("SELECT post_{$field}_gmt FROM $wpdb->posts WHERE post_status = 'publish' AND post_type IN ({$post_types}) ORDER BY post_{$field}_gmt DESC LIMIT 1");
+			break;
+		case 'blog':
+			$date = $wpdb->get_var("SELECT post_{$field} FROM $wpdb->posts WHERE post_status = 'publish' AND post_type IN ({$post_types}) ORDER BY post_{$field}_gmt DESC LIMIT 1");
+			break;
+		case 'server':
+			$add_seconds_server = date( 'Z' );
+			$date = $wpdb->get_var("SELECT DATE_ADD(post_{$field}_gmt, INTERVAL '$add_seconds_server' SECOND) FROM $wpdb->posts WHERE post_status = 'publish' AND post_type IN ({$post_types}) ORDER BY post_{$field}_gmt DESC LIMIT 1");
+			break;
+	}
+
+	if ( $date ) {
+		wp_cache_set( $key, $date, 'timeinfo' );
+
+		return $date;
+	}
+
+	return false;
 }
 
 /**
diff --git a/tests/phpunit/tests/comment/lastCommentModified.php b/tests/phpunit/tests/comment/lastCommentModified.php
new file mode 100644
index 0000000000..24d0d6c7f1
--- /dev/null
+++ b/tests/phpunit/tests/comment/lastCommentModified.php
@@ -0,0 +1,118 @@
+<?php
+
+/**
+ * @group comment
+ * @group 38027
+ */
+class Tests_Comment_Last_Modified extends WP_UnitTestCase {
+	public function test_no_comments() {
+		$this->assertFalse( get_lastcommentmodified() );
+	}
+
+	public function test_default_timezone() {
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-01 11:00:00',
+			'comment_date_gmt' => '2000-01-01 10:00:00',
+		) );
+
+		$this->assertSame( strtotime( '2000-01-01 10:00:00' ), strtotime( get_lastcommentmodified() ) );
+	}
+
+	public function test_server_timezone() {
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-01 11:00:00',
+			'comment_date_gmt' => '2000-01-01 10:00:00',
+		) );
+
+		$this->assertSame( strtotime( '2000-01-01 10:00:00' ), strtotime( get_lastcommentmodified() ) );
+	}
+
+	public function test_blog_timezone() {
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-01 11:00:00',
+			'comment_date_gmt' => '2000-01-01 10:00:00',
+		) );
+
+		$this->assertSame( '2000-01-01 11:00:00', get_lastcommentmodified( 'blog' ) );
+	}
+
+	public function test_gmt_timezone() {
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-01 11:00:00',
+			'comment_date_gmt' => '2000-01-01 10:00:00',
+		) );
+
+		$this->assertSame( strtotime( '2000-01-01 10:00:00' ), strtotime( get_lastcommentmodified( 'GMT' ) ) );
+	}
+
+	public function test_unknown_timezone() {
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-01 11:00:00',
+			'comment_date_gmt' => '2000-01-01 10:00:00',
+		) );
+
+		$this->assertFalse( get_lastcommentmodified( 'foo' ) );
+	}
+
+	public function test_data_is_cached() {
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2015-04-01 11:00:00',
+			'comment_date_gmt' => '2015-04-01 10:00:00',
+		) );
+
+		get_lastcommentmodified();
+		$this->assertSame( strtotime( '2015-04-01 10:00:00' ), strtotime( wp_cache_get( 'lastcommentmodified:server', 'timeinfo' ) ) );
+	}
+
+	public function test_cache_is_cleared() {
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-01 11:00:00',
+			'comment_date_gmt' => '2000-01-01 10:00:00',
+		) );
+
+		get_lastcommentmodified();
+
+		$this->assertSame( strtotime( '2000-01-01 10:00:00' ), strtotime( wp_cache_get( 'lastcommentmodified:server', 'timeinfo' ) ) );
+
+		self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-02 11:00:00',
+			'comment_date_gmt' => '2000-01-02 10:00:00',
+		) );
+
+		$this->assertFalse( wp_cache_get( 'lastcommentmodified:server', 'timeinfo' ) );
+		$this->assertSame( strtotime( '2000-01-02 10:00:00' ), strtotime( get_lastcommentmodified() ) );
+		 $this->assertSame( strtotime( '2000-01-02 10:00:00' ), strtotime( wp_cache_get( 'lastcommentmodified:server', 'timeinfo' ) ) );
+	}
+
+	public function test_cache_is_cleared_when_comment_is_trashed() {
+		$comment_1 = self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '1998-01-01 11:00:00',
+			'comment_date_gmt' => '1998-01-01 10:00:00',
+		) );
+
+		$comment_2 = self::factory()->comment->create_and_get( array(
+			'comment_status'   => 1,
+			'comment_date'     => '2000-01-02 11:00:00',
+			'comment_date_gmt' => '2000-01-02 10:00:00',
+		) );
+
+		get_lastcommentmodified();
+
+		$this->assertSame( strtotime( '2000-01-02 10:00:00' ), strtotime( wp_cache_get( 'lastcommentmodified:server', 'timeinfo' ) ) );
+
+		wp_trash_comment( $comment_2->comment_ID );
+
+		$this->assertFalse( wp_cache_get( 'lastcommentmodified:server', 'timeinfo' ) );
+		$this->assertSame( strtotime( '1998-01-01 10:00:00' ), strtotime( get_lastcommentmodified() ) );
+		$this->assertSame( strtotime( '1998-01-01 10:00:00' ), strtotime( wp_cache_get( 'lastcommentmodified:server', 'timeinfo' ) ) );
+	}
+}