diff --git a/core/modules/config_translation/src/Tests/ConfigTranslationUiTest.php b/core/modules/config_translation/src/Tests/ConfigTranslationUiTest.php
index a05f7e7..f220d1a 100644
--- a/core/modules/config_translation/src/Tests/ConfigTranslationUiTest.php
+++ b/core/modules/config_translation/src/Tests/ConfigTranslationUiTest.php
@@ -485,7 +485,7 @@ public function testDateFormatTranslation() {
 
       // Formatting the date 8 / 27 / 1985 @ 13:37 EST with pattern D should
       // display "Tue".
-      $formatted_date = format_date(494015820, $id, NULL, NULL, 'fr');
+      $formatted_date = format_date(494015820, $id, NULL, 'America/New_York', 'fr');
       $this->assertEqual($formatted_date, 'Tue', 'Got the right formatted date using the date format translation pattern.');
     }
   }
diff --git a/core/modules/locale/src/Tests/LocaleConfigTranslationTest.php b/core/modules/locale/src/Tests/LocaleConfigTranslationTest.php
index 39fcccd..588e779 100644
--- a/core/modules/locale/src/Tests/LocaleConfigTranslationTest.php
+++ b/core/modules/locale/src/Tests/LocaleConfigTranslationTest.php
@@ -117,7 +117,7 @@ public function testConfigTranslation() {
 
     // Formatting the date 8 / 27 / 1985 @ 13:37 EST with pattern D should
     // display "Tue".
-    $formatted_date = format_date(494015820, $type = 'medium', NULL, NULL, $this->langcode);
+    $formatted_date = format_date(494015820, $type = 'medium', NULL, 'America/New_York', $this->langcode);
     $this->assertEqual($formatted_date, 'Tue', 'Got the right formatted date using the date format translation pattern.');
 
     // Assert strings from image module config are not available.
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index cc3b270..66fe87e 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -831,6 +831,12 @@ protected function initConfig(ContainerInterface $container) {
       ->set('css.preprocess', FALSE)
       ->set('js.preprocess', FALSE)
       ->save();
+
+    // Set an explicit time zone to not rely on the system one, which may vary
+    // from setup to setup.
+    $config->getEditable('system.date')
+      ->set('timezone.default', 'Australia/Sydney')
+      ->save();
   }
 
   /**