commit e52aaf1e6c2332c4c9f97fd9004a8f10f573bfa1
Author: Dries <dries@buytaert.net>
Date:   Fri Sep 5 12:57:27 2014 -0400

    Issue #2241275 by Wim Leers: DateFormat cache tag: don't set the cache tag, but invalidate the entire render cache?.

diff --git a/core/lib/Drupal/Core/Datetime/Entity/DateFormat.php b/core/lib/Drupal/Core/Datetime/Entity/DateFormat.php
index ed40b90..2cc01e8 100644
--- a/core/lib/Drupal/Core/Datetime/Entity/DateFormat.php
+++ b/core/lib/Drupal/Core/Datetime/Entity/DateFormat.php
@@ -91,4 +91,18 @@ public static function sort(ConfigEntityInterface $a, ConfigEntityInterface $b)
     return $a->isLocked() ? 1 : -1;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheTag() {
+    return array('rendered' => TRUE);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getListCacheTags() {
+    return array('rendered' => TRUE);
+  }
+
 }