diff --git a/core/modules/datetime/tests/src/Functional/DateTimeFieldTest.php b/core/modules/datetime/tests/src/Functional/DateTimeFieldTest.php
index 54129902003e56ab0d5f45d5f63245b4b628dcfd..f4606d49686c10397499012d2599b8b2e90e7185 100644
--- a/core/modules/datetime/tests/src/Functional/DateTimeFieldTest.php
+++ b/core/modules/datetime/tests/src/Functional/DateTimeFieldTest.php
@@ -44,6 +44,7 @@ public function testDateField() {
     foreach (static::$timezones as $timezone) {
 
       $this->setSiteTimezone($timezone);
+      $this->assertEquals($timezone, $this->config('system.date')->get('timezone.default'), 'Time zone set to ' . $timezone);
 
       // Display creation form.
       $this->drupalGet('entity_test/add');
@@ -117,7 +118,12 @@ public function testDateField() {
               $expected_iso = format_date($date->getTimestamp(), 'custom', 'Y-m-d\TH:i:s\Z', DATETIME_STORAGE_TIMEZONE);
               $output = $this->renderTestEntity($id);
               $expected_markup = '<time datetime="' . $expected_iso . '" class="datetime">' . $expected . '</time>';
-              $this->assertContains($expected_markup, $output, SafeMarkup::format('Formatted date field using %value format displayed as %expected with %expected_iso attribute.', ['%value' => $new_value, '%expected' => $expected, '%expected_iso' => $expected_iso]));
+              $this->assertContains($expected_markup, $output, new FormattableMarkup('Formatted date field using %value format displayed as %expected with %expected_iso attribute in %timezone.', [
+                '%value' => $new_value,
+                '%expected' => $expected,
+                '%expected_iso' => $expected_iso,
+                '%timezone' => $timezone,
+              ]));
               break;
           }
         }
@@ -131,7 +137,10 @@ public function testDateField() {
         ->save();
       $expected = $date->format(DATETIME_DATE_STORAGE_FORMAT);
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, SafeMarkup::format('Formatted date field using plain format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using plain format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
 
       // Verify that the 'datetime_custom' formatter works.
       $this->displayOptions['type'] = 'datetime_custom';
@@ -141,7 +150,10 @@ public function testDateField() {
         ->save();
       $expected = $date->format($this->displayOptions['settings']['date_format']);
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, SafeMarkup::format('Formatted date field using datetime_custom format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using datetime_custom format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
 
       // Test that allowed markup in custom format is preserved and XSS is
       // removed.
@@ -151,7 +163,10 @@ public function testDateField() {
         ->save();
       $expected = '<strong>' . $date->format('m/d/Y') . '</strong>alert(String.fromCharCode(88,83,83))';
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
 
       // Verify that the 'datetime_time_ago' formatter works for intervals in the
       // past.  First update the test entity so that the date difference always
@@ -178,7 +193,10 @@ public function testDateField() {
         '@interval' => $this->dateFormatter->formatTimeDiffSince($timestamp, ['granularity' => $this->displayOptions['settings']['granularity']])
       ]);
       $output = $this->renderTestEntity($id);
-      $this->assertContains((string) $expected, $output, SafeMarkup::format('Formatted date field using datetime_time_ago format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains((string) $expected, $output, new FormattableMarkup('Formatted date field using datetime_time_ago format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
 
       // Verify that the 'datetime_time_ago' formatter works for intervals in the
       // future.  First update the test entity so that the date difference always
@@ -199,7 +217,10 @@ public function testDateField() {
         '@interval' => $this->dateFormatter->formatTimeDiffUntil($timestamp, ['granularity' => $this->displayOptions['settings']['granularity']])
       ]);
       $output = $this->renderTestEntity($id);
-      $this->assertContains((string) $expected, $output, SafeMarkup::format('Formatted date field using datetime_time_ago format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains((string) $expected, $output, new FormattableMarkup('Formatted date field using datetime_time_ago format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
     }
   }
 
@@ -646,6 +667,7 @@ public function testDefaultValue() {
     foreach (static::$timezones as $timezone) {
 
       $this->setSiteTimezone($timezone);
+      $this->assertEquals($timezone, $this->config('system.date')->get('timezone.default'), 'Time zone set to ' . $timezone);
 
       // Set now as default_value.
       $field_edit = [
diff --git a/core/modules/datetime_range/tests/src/Functional/DateRangeFieldTest.php b/core/modules/datetime_range/tests/src/Functional/DateRangeFieldTest.php
index e7f0fbbd478e0f69c8a1362e02f467e9db9b62af..50f61ee9c0d42b2ee3343b1195807806e436bd6e 100644
--- a/core/modules/datetime_range/tests/src/Functional/DateRangeFieldTest.php
+++ b/core/modules/datetime_range/tests/src/Functional/DateRangeFieldTest.php
@@ -53,6 +53,7 @@ public function testDateRangeField() {
     foreach (static::$timezones as $timezone) {
 
       $this->setSiteTimezone($timezone);
+      $this->assertEquals($timezone, $this->config('system.date')->get('timezone.default'), 'Time zone set to ' . $timezone);
 
       // Ensure field is set to a date-only field.
       $this->fieldStorage->setSetting('datetime_type', DateRangeItem::DATETIME_TYPE_DATE);
@@ -134,15 +135,17 @@ public function testDateRangeField() {
       $end_expected_iso = $this->dateFormatter->format($end_date->getTimestamp(), 'custom', 'Y-m-d\TH:i:s\Z', DATETIME_STORAGE_TIMEZONE);
       $end_expected_markup = '<time datetime="' . $end_expected_iso . '" class="datetime">' . $end_expected . '</time>';
       $output = $this->renderTestEntity($id);
-      $this->assertContains($start_expected_markup, $output, new FormattableMarkup('Formatted date field using %value format displayed as %expected with %expected_iso attribute.', [
+      $this->assertContains($start_expected_markup, $output, new FormattableMarkup('Formatted date field using %value format displayed as %expected with %expected_iso attribute in %timezone.', [
         '%value' => 'long',
         '%expected' => $start_expected,
         '%expected_iso' => $start_expected_iso,
+        '%timezone' => $timezone,
       ]));
-      $this->assertContains($end_expected_markup, $output, new FormattableMarkup('Formatted date field using %value format displayed as %expected with %expected_iso attribute.', [
+      $this->assertContains($end_expected_markup, $output, new FormattableMarkup('Formatted date field using %value format displayed as %expected with %expected_iso attribute in %timezone.', [
         '%value' => 'long',
         '%expected' => $end_expected,
         '%expected_iso' => $end_expected_iso,
+        '%timezone' => $timezone,
       ]));
       $this->assertContains(' THESEPARATOR ', $output, 'Found proper separator');
 
@@ -159,7 +162,10 @@ public function testDateRangeField() {
         ->save();
       $expected = $start_date->format(DATETIME_DATE_STORAGE_FORMAT) . ' - ' . $end_date->format(DATETIME_DATE_STORAGE_FORMAT);
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using plain format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using plain format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
 
       // Verify that the custom formatter works.
       $this->displayOptions['type'] = 'daterange_custom';
@@ -169,7 +175,10 @@ public function testDateRangeField() {
         ->save();
       $expected = $start_date->format($this->displayOptions['settings']['date_format']) . ' - ' . $end_date->format($this->displayOptions['settings']['date_format']);
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
 
       // Test that allowed markup in custom format is preserved and XSS is
       // removed.
@@ -179,7 +188,10 @@ public function testDateRangeField() {
         ->save();
       $expected = '<strong>' . $start_date->format('m/d/Y') . '</strong>alert(String.fromCharCode(88,83,83)) - <strong>' . $end_date->format('m/d/Y') . '</strong>alert(String.fromCharCode(88,83,83))';
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
 
       // Test formatters when start date and end date are the same
       $this->drupalGet('entity_test/add');
@@ -218,12 +230,13 @@ public function testDateRangeField() {
       $start_expected_iso = $this->dateFormatter->format($start_date->getTimestamp(), 'custom', 'Y-m-d\TH:i:s\Z', DATETIME_STORAGE_TIMEZONE);
       $start_expected_markup = '<time datetime="' . $start_expected_iso . '" class="datetime">' . $start_expected . '</time>';
       $output = $this->renderTestEntity($id);
-      $this->assertContains($start_expected_markup, $output, new FormattableMarkup('Formatted date field using %value format displayed as %expected with %expected_iso attribute.', [
+      $this->assertContains($start_expected_markup, $output, new FormattableMarkup('Formatted date field using %value format displayed as %expected with %expected_iso attribute in %timezone.', [
         '%value' => 'long',
         '%expected' => $start_expected,
         '%expected_iso' => $start_expected_iso,
+        '%timezone' => $timezone,
       ]));
-      $this->assertNotContains(' THESEPARATOR ', $output, 'Separator not found on page');
+      $this->assertNotContains(' THESEPARATOR ', $output, 'Separator not found on page in ' . $timezone);
 
       // Verify that hook_entity_prepare_view can add attributes.
       // @see entity_test_entity_prepare_view()
@@ -237,7 +250,10 @@ public function testDateRangeField() {
         ->save();
       $expected = $start_date->format(DATETIME_DATE_STORAGE_FORMAT);
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using plain format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using plain format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
       $this->assertNotContains(' THESEPARATOR ', $output, 'Separator not found on page');
 
       $this->displayOptions['type'] = 'daterange_custom';
@@ -247,7 +263,10 @@ public function testDateRangeField() {
         ->save();
       $expected = $start_date->format($this->displayOptions['settings']['date_format']);
       $output = $this->renderTestEntity($id);
-      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected.', ['%expected' => $expected]));
+      $this->assertContains($expected, $output, new FormattableMarkup('Formatted date field using daterange_custom format displayed as %expected in %timezone.', [
+        '%expected' => $expected,
+        '%timezone' => $timezone,
+      ]));
       $this->assertNotContains(' THESEPARATOR ', $output, 'Separator not found on page');
     }
   }