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/rdf/src/CommonDataConverter.php b/core/modules/rdf/src/CommonDataConverter.php
index af95d37..ef63439 100644
--- a/core/modules/rdf/src/CommonDataConverter.php
+++ b/core/modules/rdf/src/CommonDataConverter.php
@@ -34,7 +34,7 @@ public static function rawValue($data) {
    *   Returns the ISO 8601 timestamp.
    */
   public static function dateIso8601Value($data) {
-    return date_iso8601($data['value']);
+    return \Drupal::service('date.formatter')->format($data['value'], 'custom', 'c', 'UTC');
   }
 
 }
diff --git a/core/modules/rdf/src/Tests/CommentAttributesTest.php b/core/modules/rdf/src/Tests/CommentAttributesTest.php
index e678de2..f7ad33a 100644
--- a/core/modules/rdf/src/Tests/CommentAttributesTest.php
+++ b/core/modules/rdf/src/Tests/CommentAttributesTest.php
@@ -264,14 +264,14 @@ function _testBasicCommentRdfaMarkup($graph, CommentInterface $comment, $account
     // Comment date.
     $expected_value = array(
       'type' => 'literal',
-      'value' => date('c', $comment->getCreatedTime()),
+      'value' => format_date($comment->getCreatedTime(), 'custom', 'c', 'UTC'),
       'datatype' => 'http://www.w3.org/2001/XMLSchema#dateTime',
     );
     $this->assertTrue($graph->hasProperty($comment_uri, 'http://purl.org/dc/terms/date', $expected_value), 'Comment date found in RDF output (dc:date).');
     // Comment date.
     $expected_value = array(
       'type' => 'literal',
-      'value' => date('c', $comment->getCreatedTime()),
+      'value' => format_date($comment->getCreatedTime(), 'custom', 'c', 'UTC'),
       'datatype' => 'http://www.w3.org/2001/XMLSchema#dateTime',
     );
     $this->assertTrue($graph->hasProperty($comment_uri, 'http://purl.org/dc/terms/created', $expected_value), 'Comment date found in RDF output (dc:created).');
diff --git a/core/modules/rdf/src/Tests/StandardProfileTest.php b/core/modules/rdf/src/Tests/StandardProfileTest.php
index 784bd80..5036c62 100644
--- a/core/modules/rdf/src/Tests/StandardProfileTest.php
+++ b/core/modules/rdf/src/Tests/StandardProfileTest.php
@@ -356,7 +356,7 @@ protected function assertRdfaCommonNodeProperties($graph, NodeInterface $node, $
     // Created date.
     $expected_value = array(
       'type' => 'literal',
-      'value' => date_iso8601($node->get('created')->value),
+      'value' => format_date($node->get('created')->value, 'custom', 'c', 'UTC'),
       'lang' => 'en',
     );
     $this->assertTrue($graph->hasProperty($uri, 'http://schema.org/dateCreated', $expected_value), "$message_prefix created date was found (schema:dateCreated) in teaser.");
@@ -445,7 +445,7 @@ protected function assertRdfaNodeCommentProperties($graph) {
     // Comment created date.
     $expected_value = array(
       'type' => 'literal',
-      'value' => date_iso8601($this->articleComment->get('created')->value),
+      'value' => format_date($this->articleComment->get('created')->value, 'custom', 'c', 'UTC'),
       'lang' => 'en',
     );
     $this->assertTrue($graph->hasProperty($this->articleCommentUri, 'http://schema.org/dateCreated', $expected_value), 'Article comment created date was found (schema:dateCreated).');
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 30aa69e..b4ebe73 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -831,6 +831,15 @@ 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. The Australia/Sydney time zone is chosen so all
+    // tests are run using an edge case scenario (UTC+10 and DST). This choice
+    // is made to prevent time zone related regressions and reduce the
+    // fragility of the testing system in general.
+    $config->getEditable('system.date')
+      ->set('timezone.default', 'Australia/Sydney')
+      ->save();
   }
 
   /**