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() {
 
   $default_headers = array(
     'Expires' => 'Sun, 19 Nov 1978 05:00:00 GMT',
-    'Last-Modified' => gmdate(DATE_RFC1123, REQUEST_TIME),
+    'Last-Modified' => gmdate('D, d M Y H:i:s', REQUEST_TIME) . ' GMT',
     'Cache-Control' => 'no-cache, must-revalidate, post-check=0, pre-check=0',
     'ETag' => '"' . REQUEST_TIME . '"',
   );
diff --git a/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php b/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
index 28d024b..faab095 100644
--- a/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
+++ b/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
@@ -41,7 +41,7 @@ public function testFeed($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);