diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index b125ae8..63e8deb 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -845,7 +845,7 @@ function drupal_page_header() {
 
   $default_headers = array(
     'Expires' => 'Sun, 19 Nov 1978 05:00:00 GMT',
-    'Last-Modified' => gmdate(DATE_RFC1123, REQUEST_TIME),
+    'Last-Modified' => gmdate('D, d M Y H:i:s', REQUEST_TIME) . ' GMT',
     'Cache-Control' => 'no-cache, must-revalidate, post-check=0, pre-check=0',
     'ETag' => '"' . REQUEST_TIME . '"',
   );
diff --git a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
index 3063bb2..d07c716 100644
--- a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
@@ -174,7 +174,7 @@ protected function setResponseCacheable(Response $response, Request $request) {
     // Last-Modified and an ETag header on the response.
     if (!$response->headers->has('Last-Modified')) {
       $timestamp = REQUEST_TIME;
-      $response->setLastModified(new \DateTime(gmdate(DATE_RFC1123, REQUEST_TIME)));
+      $response->setLastModified(new \DateTime(gmdate('D, d M Y H:i:s', REQUEST_TIME) . ' GMT'));
     }
     else {
       $timestamp = $response->getLastModified()->getTimestamp();
diff --git a/core/modules/aggregator/src/Plugin/aggregator/fetcher/DefaultFetcher.php b/core/modules/aggregator/src/Plugin/aggregator/fetcher/DefaultFetcher.php
index 4e8eee9..0cc5d77 100644
--- a/core/modules/aggregator/src/Plugin/aggregator/fetcher/DefaultFetcher.php
+++ b/core/modules/aggregator/src/Plugin/aggregator/fetcher/DefaultFetcher.php
@@ -65,7 +65,7 @@ public function fetch(FeedInterface $feed) {
       $request->addHeader('If-None-Match', $feed->getEtag());
     }
     if ($feed->getLastModified()) {
-      $request->addHeader('If-Modified-Since', gmdate(DATE_RFC1123, $feed->getLastModified()));
+      $request->addHeader('If-Modified-Since', gmdate('D, d M Y H:i:s', $feed->getLastModified()). ' GMT');
     }
 
     try {
diff --git a/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php b/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
index 28d024b..faab095 100644
--- a/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
+++ b/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
@@ -41,7 +41,7 @@ public function testFeed($use_last_modified, $use_etag, Request $request) {
     // Send appropriate response. We respond with a 304 not modified on either
     // etag or on last modified.
     if ($use_last_modified) {
-      $response->headers->set('Last-Modified', gmdate(DATE_RFC1123, $last_modified));
+      $response->headers->set('Last-Modified', gmdate('D, d M Y H:i:s', $last_modified) . ' GMT');
     }
     if ($use_etag) {
       $response->headers->set('ETag', $etag);
diff --git a/core/modules/system/src/Tests/Bootstrap/PageCacheTest.php b/core/modules/system/src/Tests/Bootstrap/PageCacheTest.php
index fdf16b9..9945686 100644
--- a/core/modules/system/src/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/src/Tests/Bootstrap/PageCacheTest.php
@@ -141,7 +141,7 @@ function testConditionalRequests() {
     $this->assertResponse(200, 'Conditional request without If-None-Match returned 200 OK.');
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
 
-    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate(DATE_RFC1123, strtotime($last_modified) + 1), 'If-None-Match: ' . $etag));
+    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate('D, d M Y H:i:s', strtotime($last_modified) + 1) . ' GMT', 'If-None-Match: ' . $etag));
     $this->assertResponse(200, 'Conditional request with new a If-Modified-Since date newer than Last-Modified returned 200 OK.');
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
 
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index 3599c59..4059978 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -138,7 +138,7 @@ function _toolbar_initialize_page_cache() {
   // accesses the callback URL again (e.g., after clearing the browser cache or
   // when force-reloading a Drupal page).
   $max_age = 3600 * 24 * 365;
-  drupal_add_http_header('Expires', gmdate(DATE_RFC1123, REQUEST_TIME + $max_age));
+  drupal_add_http_header('Expires', gmdate('D, d M Y H:i:s', REQUEST_TIME + $max_age) . ' GMT');
   drupal_add_http_header('Cache-Control', 'private, max-age=' . $max_age);
 }