diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 63e8deb..00ee682 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -4,6 +4,7 @@
  * Functions that need to be loaded on every Drupal request.
  */
 
+use Drupal\Component\Datetime\DateTimePlus;
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\Environment;
 use Drupal\Component\Utility\NestedArray;
@@ -845,7 +846,7 @@ function drupal_page_header() {
 
   $default_headers = array(
     'Expires' => 'Sun, 19 Nov 1978 05:00:00 GMT',
-    'Last-Modified' => gmdate('D, d M Y H:i:s', REQUEST_TIME) . ' GMT',
+    'Last-Modified' => gmdate(DateTimePlus::RFC2616, REQUEST_TIME),
     'Cache-Control' => 'no-cache, must-revalidate, post-check=0, pre-check=0',
     'ETag' => '"' . REQUEST_TIME . '"',
   );
@@ -867,7 +868,7 @@ function drupal_serve_page_from_cache(Response $response, Request $request) {
   // Only allow caching in the browser and prevent that the response is stored
   // by an external proxy server when the following conditions apply:
   // 1. There is a session cookie on the request.
-  // 2. The Vary: Cookie header is on the response.
+  // 2. The Vary: Cookie header is on the response.q
   // 3. The Cache-Control header does not contain the no-cache directive.
   if ($request->cookies->has(session_name()) &&
     in_array('Cookie', $response->getVary()) &&
diff --git a/core/lib/Drupal/Component/Datetime/DateTimePlus.php b/core/lib/Drupal/Component/Datetime/DateTimePlus.php
index 4c4122c..7ce8c04 100644
--- a/core/lib/Drupal/Component/Datetime/DateTimePlus.php
+++ b/core/lib/Drupal/Component/Datetime/DateTimePlus.php
@@ -31,6 +31,13 @@ class DateTimePlus extends \DateTime {
   const FORMAT   = 'Y-m-d H:i:s';
 
   /**
+   * A RFC2616 compliant subset of RFC1123.
+   *
+   * Example: Sun, 06 Nov 1994 08:49:37 GMT
+   */
+  const RFC2616 = 'D, d M Y H:i:s \G\M\T';
+
+  /**
    * An array of possible date parts.
    */
   protected static $dateParts = array(
diff --git a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
index d07c716..aef1910 100644
--- a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\EventSubscriber;
 
+use Drupal\Component\Datetime\DateTimePlus;
 use Drupal\Core\Config\Config;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Language\Language;
@@ -174,7 +175,7 @@ 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('D, d M Y H:i:s', REQUEST_TIME) . ' GMT'));
+      $response->setLastModified(new \DateTime(gmdate(DateTimePlus::RFC2616, REQUEST_TIME)));
     }
     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 0cc5d77..0cec424 100644
--- a/core/modules/aggregator/src/Plugin/aggregator/fetcher/DefaultFetcher.php
+++ b/core/modules/aggregator/src/Plugin/aggregator/fetcher/DefaultFetcher.php
@@ -9,6 +9,7 @@
 
 use Drupal\aggregator\Plugin\FetcherInterface;
 use Drupal\aggregator\FeedInterface;
+use Drupal\Component\Datetime\DateTimePlus;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Exception\RequestException;
@@ -65,7 +66,7 @@ public function fetch(FeedInterface $feed) {
       $request->addHeader('If-None-Match', $feed->getEtag());
     }
     if ($feed->getLastModified()) {
-      $request->addHeader('If-Modified-Since', gmdate('D, d M Y H:i:s', $feed->getLastModified()). ' GMT');
+      $request->addHeader('If-Modified-Since', gmdate(DateTimePlus::RFC2616, $feed->getLastModified()));
     }
 
     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 faab095..c1d04c5 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
@@ -6,6 +6,7 @@
 
 namespace Drupal\aggregator_test\Controller;
 
+use Drupal\Component\Datetime\DateTimePlus;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Component\Utility\Crypt;
 use Symfony\Component\HttpFoundation\Response;
@@ -41,7 +42,7 @@ 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('D, d M Y H:i:s', $last_modified) . ' GMT');
+      $response->headers->set('Last-Modified', gmdate(DateTimePlus::RFC2616, $last_modified));
     }
     if ($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 9945686..38dfe96 100644
--- a/core/modules/system/src/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/src/Tests/Bootstrap/PageCacheTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\system\Tests\Bootstrap;
 
+use Drupal\Component\Datetime\DateTimePlus;
 use Symfony\Component\Routing\RequestContext;
 use Drupal\simpletest\WebTestBase;
 use Drupal\Core\Cache\Cache;
@@ -141,7 +142,7 @@ 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.');
 
-    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate('D, d M Y H:i:s', strtotime($last_modified) + 1) . ' GMT', 'If-None-Match: ' . $etag));
+    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate(DateTimePlus::RFC2616, strtotime($last_modified) + 1), '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 4059978..f3c42e6 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('D, d M Y H:i:s', REQUEST_TIME + $max_age) . ' GMT');
+  drupal_add_http_header('Expires', gmdate(DateTimePlus::RFC2616, REQUEST_TIME + $max_age));
   drupal_add_http_header('Cache-Control', 'private, max-age=' . $max_age);
 }