diff --git a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
index 6126cd0..0b9bbad 100644
--- a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
@@ -61,8 +61,7 @@ public function onRespond(FilterResponseEvent $event) {
     // since the page is in fact being regenerated right now.
     // @todo Remove this and use a more intelligent default so that HTTP
     // caching can function properly.
-    // @todo use $response->setLastModified()
-    $response->headers->set('Last-Modified', gmdate(DATE_RFC1123, REQUEST_TIME));
+    $response->setLastModified(new \DateTime(gmdate(DATE_RFC1123, REQUEST_TIME)));
 
     // Also give each page a unique ETag. This will force clients to include
     // both an If-Modified-Since header and an If-None-Match header when doing