diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index e15f57b..20394da 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -249,6 +249,15 @@ define('REGISTRY_WRITE_LOOKUP_CACHE', 2);
 define('DRUPAL_PHP_FUNCTION_PATTERN', '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*');
 
 /**
+ * A RFC7231 Compliant date.
+ *
+ * http://tools.ietf.org/html/rfc7231#section-7.1.1.1
+ *
+ * Example: Sun, 06 Nov 1994 08:49:37 GMT
+ */
+define('DATE_RFC7231', 'D, d M Y H:i:s \G\M\T');
+
+/**
  * Provides a caching wrapper to be used in place of large array structures.
  *
  * This class should be extended by systems that need to cache large amounts
@@ -1266,7 +1275,7 @@ 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(DATE_RFC7231, REQUEST_TIME),
     'Cache-Control' => 'no-cache, must-revalidate, post-check=0, pre-check=0',
     'ETag' => '"' . REQUEST_TIME . '"',
   );
@@ -1336,7 +1345,7 @@ function drupal_serve_page_from_cache(stdClass $cache) {
     drupal_add_http_header($name, $value);
   }
 
-  $default_headers['Last-Modified'] = gmdate(DATE_RFC1123, $cache->created);
+  $default_headers['Last-Modified'] = gmdate(DATE_RFC7231, $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/aggregator/aggregator.fetcher.inc b/modules/aggregator/aggregator.fetcher.inc
index 0f72877..ed77bb6 100644
--- a/modules/aggregator/aggregator.fetcher.inc
+++ b/modules/aggregator/aggregator.fetcher.inc
@@ -27,7 +27,7 @@ function aggregator_aggregator_fetch($feed) {
     $headers['If-None-Match'] = $feed->etag;
   }
   if ($feed->modified) {
-    $headers['If-Modified-Since'] = gmdate(DATE_RFC1123, $feed->modified);
+    $headers['If-Modified-Since'] = gmdate(DATE_RFC7231, $feed->modified);
   }
 
   // Request feed.
diff --git a/modules/aggregator/tests/aggregator_test.module b/modules/aggregator/tests/aggregator_test.module
index 2d26a5d..58d9818 100644
--- a/modules/aggregator/tests/aggregator_test.module
+++ b/modules/aggregator/tests/aggregator_test.module
@@ -32,7 +32,7 @@ function aggregator_test_feed($use_last_modified = FALSE, $use_etag = FALSE) {
   // Send appropriate response. We respond with a 304 not modified on either
   // etag or on last modified.
   if ($use_last_modified) {
-    drupal_add_http_header('Last-Modified', gmdate(DATE_RFC1123, $last_modified));
+    drupal_add_http_header('Last-Modified', gmdate(DATE_RFC7231, $last_modified));
   }
   if ($use_etag) {
     drupal_add_http_header('ETag', $etag);
diff --git a/modules/simpletest/tests/bootstrap.test b/modules/simpletest/tests/bootstrap.test
index 5dcde32..f723c63 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.');
 
-    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate(DATE_RFC1123, strtotime($last_modified) + 1), 'If-None-Match: ' . $etag));
+    $this->drupalGet('', array(), array('If-Modified-Since: ' . gmdate(DATE_RFC7231, strtotime($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.');