diff --git a/core/modules/datetime/src/Tests/DateTestBase.php b/core/modules/datetime/src/Tests/DateTestBase.php
index 81e7550..4065da0 100644
--- a/core/modules/datetime/src/Tests/DateTestBase.php
+++ b/core/modules/datetime/src/Tests/DateTestBase.php
@@ -178,6 +178,7 @@ protected function setSiteTimezone($timezone) {
       ->set('timezone.user.configurable', 0)
       ->set('timezone.default', $timezone)
       ->save();
+    drupal_flush_all_caches();
   }
 
 }