diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index 9f37dfc..0461254 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -1275,7 +1275,7 @@ function drupal_page_header() {
 
   $default_headers = array(
     'Expires' => 'Sun, 19 Nov 1978 05:00:00 GMT',
-    'Last-Modified' => gmdate(DATE_RFC7231, REQUEST_TIME),
+    'Last-Modified' => gmdate(DATE_RFC1123, REQUEST_TIME),
     'Cache-Control' => 'no-cache, must-revalidate, post-check=0, pre-check=0',
     'ETag' => '"' . REQUEST_TIME . '"',
   );
@@ -1345,7 +1345,7 @@ function drupal_serve_page_from_cache(stdClass $cache) {
     drupal_add_http_header($name, $value);
   }
 
-  $default_headers['Last-Modified'] = gmdate(DATE_RFC7231, $cache->created);
+  $default_headers['Last-Modified'] = gmdate(DATE_RFC1123, $cache->created);
 
   // HTTP/1.0 proxies does not support the Vary header, so prevent any caching
   // by sending an Expires date in the past. HTTP/1.1 clients ignores the
diff --git a/modules/simpletest/tests/bootstrap.test b/modules/simpletest/tests/bootstrap.test
index f723c63..5dcde32 100644
--- a/modules/simpletest/tests/bootstrap.test
+++ b/modules/simpletest/tests/bootstrap.test
@@ -144,7 +144,7 @@ class BootstrapPageCacheTestCase extends DrupalWebTestCase {
     $this->assertResponse(200, 'Conditional request without If-None-Match returned 200 OK.');
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
 
-    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate(DATE_RFC7231, strtotime($last_modified) + 1), 'If-None-Match: ' . $etag));
+    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate(DATE_RFC1123, strtotime($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.');