core/modules/hal/src/Normalizer/TimestampItemNormalizer.php         | 2 +-
 core/modules/serialization/src/Normalizer/DateTimeNormalizer.php    | 6 ------
 .../serialization/src/Normalizer/TimestampItemNormalizer.php        | 2 +-
 .../tests/src/Unit/Normalizer/DateTimeNormalizerTest.php            | 2 +-
 .../tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php       | 2 +-
 5 files changed, 4 insertions(+), 10 deletions(-)

diff --git a/core/modules/hal/src/Normalizer/TimestampItemNormalizer.php b/core/modules/hal/src/Normalizer/TimestampItemNormalizer.php
index 2cda0fa..e3d00f1 100644
--- a/core/modules/hal/src/Normalizer/TimestampItemNormalizer.php
+++ b/core/modules/hal/src/Normalizer/TimestampItemNormalizer.php
@@ -39,7 +39,7 @@ protected function normalizedFieldValues(FieldItemInterface $field_item, $format
    */
   protected function constructValue($data, $context) {
     if (!empty($data['format'])) {
-      $context['datetime_format'] = $data['format'];
+      $context['datetime_allowed_formats'] = [$data['format']];
     }
     return ['value' => $this->serializer->denormalize($data['value'], Timestamp::class, NULL, $context)];
   }
diff --git a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
index e84571a..a6353a0 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
@@ -68,12 +68,6 @@ protected function getNormalizationTimezone() {
    * {@inheritdoc}
    */
   public function denormalize($data, $class, $format = NULL, array $context = []) {
-    // First check for a provided format, and if provided, create \DateTime
-    // object using it.
-    if (!empty($context['datetime_format'])) {
-      return \DateTime::createFromFormat($context['datetime_format'], $data);
-    }
-
     // Loop through the allowed formats and create a \DateTime from the
     // input data if it matches the defined pattern. Since the formats are
     // unambiguous (i.e., they reference an absolute time with a defined time
diff --git a/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php b/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php
index cf0f8ac..9349c4e 100644
--- a/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php
@@ -39,7 +39,7 @@ public function normalize($object, $format = NULL, array $context = []) {
    */
   protected function constructValue($data, $context) {
     if (!empty($data['format'])) {
-      $context['datetime_format'] = $data['format'];
+      $context['datetime_allowed_formats'] = [$data['format']];
     }
     return ['value' => $this->serializer->denormalize($data['value'], Timestamp::class, NULL, $context)];
   }
diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php
index 570784b..00dbbb2 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php
@@ -143,7 +143,7 @@ public function providerTestDenormalizeValidFormats() {
    * @dataProvider providerTestDenormalizeUserFormats
    */
   public function testDenormalizeUserFormats($normalized, $format, $expected) {
-    $denormalized = $this->normalizer->denormalize($normalized, DateTimeInterface::class, NULL, ['datetime_format' => $format]);
+    $denormalized = $this->normalizer->denormalize($normalized, DateTimeInterface::class, NULL, ['datetime_allowed_formats' => [$format]]);
     $this->assertSame(0, $denormalized->getTimestamp() - $expected->getTimestamp());
     $this->assertEquals($expected, $denormalized);
   }
diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
index 8575501..4d01279 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
@@ -122,7 +122,7 @@ public function testDenormalize() {
 
     $context = [
       'target_instance' => $timestamp_item->reveal(),
-      'datetime_format' => \DateTime::RFC3339,
+      'datetime_allowed_formats' => [\DateTime::RFC3339],
     ];
 
     // Mock Serializer service, to assert that the Timestamp @DataType