.../EntityTest/EntityTestDateRangeTest.php            |  4 ++--
 .../EntityTest/EntityTestDatetimeTest.php             |  2 +-
 core/modules/serialization/serialization.services.yml |  8 ++++----
 .../src/Normalizer/DateTimeIso8601Normalizer.php      | 19 +++++++++++++++++++
 .../src/Normalizer/DateTimeNormalizer.php             | 11 +++++++++++
 5 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
index 6324f91..85cb43d 100644
--- a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
+++ b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
@@ -90,8 +90,8 @@ protected function getExpectedNormalizedEntity() {
     return parent::getExpectedNormalizedEntity() + [
       static::$fieldName => [
         [
-          'value' => $this->entity->get(static::$fieldName)->value,
-          'end_value' => $this->entity->get(static::$fieldName)->value,
+          'value' => '2017-03-01T20:02:00+11:00',
+          'end_value' => '2017-03-01T20:02:00+11:00',
         ],
       ],
     ];
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 ffce48c..0c75233 100644
--- a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
+++ b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
@@ -90,7 +90,7 @@ protected function getExpectedNormalizedEntity() {
     return parent::getExpectedNormalizedEntity() + [
       static::$fieldName => [
         [
-          'value' => $this->entity->get(static::$fieldName)->value,
+          'value' => '2017-03-01T20:02:00+11:00',
         ],
       ],
     ];
diff --git a/core/modules/serialization/serialization.services.yml b/core/modules/serialization/serialization.services.yml
index 537d9a7..c883410 100644
--- a/core/modules/serialization/serialization.services.yml
+++ b/core/modules/serialization/serialization.services.yml
@@ -63,11 +63,11 @@ services:
     tags:
       # Priority must be higher than serializer.normalizer.primitive_data.
       - { name: normalizer, priority: 20, bc: bc_timestamp_normalizer_unix, bc_config_name: 'serialization.settings' }
-  serializer.normalizer.datetime:
-    class: \Drupal\serialization\Normalizer\DateTimeNormalizer
+  serializer.normalizer.datetimeiso8601:
+    class: \Drupal\serialization\Normalizer\DateTimeIso8601Normalizer
     tags:
-      # Priority must be higher than serializer.normalizer.primitive_data, lower than serializer.normalizer.timestamp.
-      - { name: normalizer, priority: 15, bc: bc_timestamp_normalizer_unix, bc_config_name: 'serialization.settings' }
+      # Priority must be higher than serializer.normalizer.primitive_data.
+      - { name: normalizer, priority: 20 }
   serializer.normalizer.password_field_item:
       class: Drupal\serialization\Normalizer\NullNormalizer
       arguments: ['Drupal\Core\Field\Plugin\Field\FieldType\PasswordItem']
diff --git a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
new file mode 100644
index 0000000..32fc4e4
--- /dev/null
+++ b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace Drupal\serialization\Normalizer;
+
+use Drupal\Core\TypedData\Plugin\DataType\DateTimeIso8601;
+
+/**
+ * Converts values for the DateTimeIso8601 data type to RFC3339.
+ *
+ * @deprecated in 8.5.0, will be removed in Drupal 9.0.0, use \Drupal\serialization\Normalizer\DateTimeNormalizer instead.
+ */
+class DateTimeIso8601Normalizer extends DateTimeNormalizer {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $supportedInterfaceOrClass = DateTimeIso8601::class;
+
+}
diff --git a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
index b5f279b..b43f635 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
@@ -9,6 +9,17 @@
 
 /**
  * Converts values for datetime objects to RFC3339 and from common formats.
+ *
+ * @internal
+ *
+ * Note that this is class can become the 'serializer.normalizer.datetime'
+ * service in Drupal 9.0.0 and allow the 'serializer.normalizer.datetimeiso8601'
+ * service to be removed in Drupal 9.0.0. That is not possible today, because
+ * this class also works for \Drupal\Core\TypedData\Plugin\DataType\Timestamp
+ * objects, but those must be ignored while the 'bc_timestamp_normalizer_unix'
+ * BC flag is enabled. If this class were already an active service, it'd cause
+ * 'timestamp' fields to not have (numeric) UNIX timestamps as normalized values
+ * anymore, which would break BC.
  */
 class DateTimeNormalizer extends NormalizerBase implements DenormalizerInterface {