diff --git a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
index 3689dddbc68..3a0750113c5 100644
--- a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
+++ b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
@@ -503,4 +503,18 @@ class BrowserTestBaseTest extends BrowserTestBase {
     $this->assertTrue(file_exists($htaccess_filename), "$htaccess_filename exists");
   }
 
+  /**
+   * Tests the assumption that local time is in 'Australia/Sydney'.
+   */
+  public function testLocalTimeZone() {
+    // The 'Australia/Sydney' time zone is set in core/tests/bootstrap.php
+    $this->assertEquals('Australia/Sydney', date_default_timezone_get());
+
+    // The 'Australia/Sydney' time zone is also set in
+    // FunctionalTestSetupTrait::initConfig().
+    $config_factory = $this->container->get('config.factory');
+    $value = $config_factory->get('system.date')->get('timezone.default');
+    $this->assertEquals('Australia/Sydney', $value);
+  }
+
 }
diff --git a/core/tests/Drupal/KernelTests/KernelTestBaseTest.php b/core/tests/Drupal/KernelTests/KernelTestBaseTest.php
index 60c2ce31aeb..a503019c544 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBaseTest.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBaseTest.php
@@ -214,6 +214,14 @@ class KernelTestBaseTest extends KernelTestBase {
     $this->assertEquals('public', \Drupal::config('system.file')->get('default_scheme'));
   }
 
+  /**
+   * Tests the assumption that local time is in 'Australia/Sydney'.
+   */
+  public function testLocalTimeZone() {
+    // The 'Australia/Sydney' time zone is set in core/tests/bootstrap.php
+    $this->assertEquals('Australia/Sydney', date_default_timezone_get());
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php b/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php
index 5b1d546b519..4c289e4d78b 100644
--- a/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php
+++ b/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php
@@ -95,6 +95,14 @@ class TestSuiteBaseTest extends TestCase {
     $this->assertEmpty(array_diff_assoc($expected_tests, $stub->testFiles));
   }
 
+  /**
+   * Tests the assumption that local time is in 'Australia/Sydney'.
+   */
+  public function testLocalTimeZone() {
+    // The 'Australia/Sydney' time zone is set in core/tests/bootstrap.php
+    $this->assertEquals('Australia/Sydney', date_default_timezone_get());
+  }
+
 }
 
 /**