.../Drupal/Core/TypedData/Plugin/DataType/DateTimeIso8601.php  | 10 ++++++++--
 core/lib/Drupal/Core/TypedData/Plugin/DataType/Timestamp.php   |  7 ++++++-
 .../EntityResource/EntityTest/EntityTestDatetimeTest.php       |  3 +++
 .../src/Functional/BcTimestampNormalizerUnixTestTrait.php      |  6 +++---
 .../serialization/src/Normalizer/TimestampNormalizer.php       |  4 ++++
 5 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/core/lib/Drupal/Core/TypedData/Plugin/DataType/DateTimeIso8601.php b/core/lib/Drupal/Core/TypedData/Plugin/DataType/DateTimeIso8601.php
index 3fe189f..594166d 100644
--- a/core/lib/Drupal/Core/TypedData/Plugin/DataType/DateTimeIso8601.php
+++ b/core/lib/Drupal/Core/TypedData/Plugin/DataType/DateTimeIso8601.php
@@ -22,11 +22,17 @@ class DateTimeIso8601 extends StringData implements DateTimeInterface {
    */
   public function getDateTime() {
     if ($this->value) {
+      // Ensure that all string representations generated from this value use
+      // the site's default timezone. Otherwise, they may end up being
+      // normalized in the user's preferred timezone.
+      // @see \Drupal\Core\Datetime\DrupalDateTime::prepareTimezone()
+      // @see drupal_get_user_timezone()
+      $default_timezone = \Drupal::config('system.date')->get('timezone.default');
       if (is_array($this->value)) {
-        $datetime = DrupalDateTime::createFromArray($this->value);
+        $datetime = DrupalDateTime::createFromArray($this->value, new \DateTimeZone($default_timezone));
       }
       else {
-        $datetime = new DrupalDateTime($this->value);
+        $datetime = new DrupalDateTime($this->value, new \DateTimeZone($default_timezone));
       }
       return $datetime;
     }
diff --git a/core/lib/Drupal/Core/TypedData/Plugin/DataType/Timestamp.php b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Timestamp.php
index 60cd9b4..07b5bc5 100644
--- a/core/lib/Drupal/Core/TypedData/Plugin/DataType/Timestamp.php
+++ b/core/lib/Drupal/Core/TypedData/Plugin/DataType/Timestamp.php
@@ -27,7 +27,12 @@ class Timestamp extends IntegerData implements DateTimeInterface {
    */
   public function getDateTime() {
     if (isset($this->value)) {
-      return DrupalDateTime::createFromTimestamp($this->value);
+      // Ensure that all string representations generated from this value use
+      // the 'UTC' timezone, because the timestamp data type specifically stores
+      // UNIX timestamps, which are always in UTC. In other words: the preferred
+      // timezone for string representations of this DateTime object is UTC. It
+      // is still possible for calling code to override this preference.
+      return DrupalDateTime::createFromTimestamp($this->value, 'UTC');
     }
   }
 
diff --git a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
index c0937d7..66108da 100644
--- a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
+++ b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
@@ -71,9 +71,12 @@ public function setUp() {
   public function testWTF() {
     $value = $this->entity->field_datetime->value;
     $date = $this->entity->field_datetime->date;
+    $value_typed_data_object = $this->entity->field_datetime->first()->getProperties()['value'];
 
     $this->assertEquals(static::$dateString, $value);
     $this->assertEquals(static::$dateString . '+00:00', $date->format('c'));
+    $this->assertEquals(static::$dateString . '+11:00', $value_typed_data_object->getDateTime()->format('c'));
+    $this->assertEquals('2017-03-01T09:02:00+00:00', $value_typed_data_object->getDateTime()->setTimezone(new \DateTimeZone('UTC'))->format('c'));
   }
 
   /**
diff --git a/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php b/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php
index 3ba5d32..506db71 100644
--- a/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php
+++ b/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php
@@ -29,11 +29,11 @@ protected function formatExpectedTimestampItemValues($timestamp) {
 
     // Otherwise, format the date string to the same that
     // \Drupal\serialization\Normalizer\TimestampItemNormalizer will produce.
-    // 'Australia/Sydney' is the default time zone for tests. It is hardcoded
-    // here for test cases where the config isn't available.
     $date = new \DateTime();
     $date->setTimestamp($timestamp);
-    $date->setTimezone(new \DateTimeZone('Australia/Sydney'));
+    // Per \Drupal\Core\TypedData\Plugin\DataType\Timestamp::getDateTime(), they
+    // default to string representations in the UTC timezone.
+    $date->setTimezone(new \DateTimeZone('UTC'));
 
     // Format is also added to the expected return values.
     return [
diff --git a/core/modules/serialization/src/Normalizer/TimestampNormalizer.php b/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
index 123f105..eac3b97 100644
--- a/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
@@ -6,6 +6,10 @@
 
 /**
  * Converts values for the Timestamp data type to and from common formats.
+ *
+ * Note that \Drupal\Core\TypedData\Plugin\DataType\Timestamp::getDateTime()
+ * explicitly sets a default timezone of UTC. This ensures the string
+ * representation generated by DateTimeNormalizer::normalize() is also in UTC.
  */
 class TimestampNormalizer extends DateTimeNormalizer {