.../serialization/serialization.services.yml       |  5 ++-
 .../src/Normalizer/TimestampItemNormalizer.php     | 23 +++----------
 .../src/Normalizer/TimestampNormalizer.php         | 39 ++++++++++++++++++++--
 .../Normalizer/TimestampItemNormalizerTest.php     |  6 ++--
 4 files changed, 48 insertions(+), 25 deletions(-)

diff --git a/core/modules/serialization/serialization.services.yml b/core/modules/serialization/serialization.services.yml
index 957770a..bd130ae 100644
--- a/core/modules/serialization/serialization.services.yml
+++ b/core/modules/serialization/serialization.services.yml
@@ -54,7 +54,10 @@ services:
       - { name: normalizer, priority: 9 }
   serializer.normalizer.timestamp_item:
     class: Drupal\serialization\Normalizer\TimestampItemNormalizer
-    deprecated: The "%service_id% service is deprecated in 8.5.0, use the serializer.normalizer.timestamp service instead.
+    tags:
+      # Priority must be higher than serializer.normalizer.field_item and lower
+      # than hal normalizers.
+      - { name: normalizer, priority: 8, bc: bc_timestamp_normalizer_unix, bc_config_name: 'serialization.settings' }
   serializer.normalizer.timestamp:
     class: Drupal\serialization\Normalizer\TimestampNormalizer
     tags:
diff --git a/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php b/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php
index 5b6c8a9..46e7be0 100644
--- a/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php
@@ -3,17 +3,15 @@
 namespace Drupal\serialization\Normalizer;
 
 use Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem;
-use Symfony\Component\Serializer\Exception\InvalidArgumentException;
+use Drupal\Core\TypedData\Plugin\DataType\Timestamp;
 
 /**
  * Converts values for TimestampItem to and from common formats.
  *
- * @deprecated in 8.5.0, use \Drupal\serialization\Normalizer\TimestampNormalizer instead.
+ * Overrides FieldItemNormalizer to use\Drupal\serialization\Normalizer\TimestampNormalizer
  */
 class TimestampItemNormalizer extends FieldItemNormalizer {
 
-  use TimeStampItemNormalizerTrait;
-
   /**
    * The interface or class that this Normalizer supports.
    *
@@ -24,21 +22,8 @@ class TimestampItemNormalizer extends FieldItemNormalizer {
   /**
    * {@inheritdoc}
    */
-  public function normalize($field_item, $format = NULL, array $context = []) {
-    $data = parent::normalize($field_item, $format, $context);
-
-    return $this->processNormalizedValues($data);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
-    if (empty($data['value'])) {
-      throw new InvalidArgumentException('No "value" attribute present');
-    }
-
-    return parent::denormalize($data, $class, $format, $context);
+  protected function constructValue($data, $context) {
+    return $this->serializer->denormalize($data, Timestamp::class, NULL, $context);
   }
 
 }
diff --git a/core/modules/serialization/src/Normalizer/TimestampNormalizer.php b/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
index 81b5495..79fb725 100644
--- a/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
@@ -3,11 +3,13 @@
 namespace Drupal\serialization\Normalizer;
 
 use Drupal\Core\TypedData\Plugin\DataType\Timestamp;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
 
 /**
  * Converts values for the Timestamp data type to and from common formats.
  */
-class TimestampNormalizer extends NormalizerBase {
+class TimestampNormalizer extends NormalizerBase implements DenormalizerInterface {
 
   /**
    * Allowed timestamps formats for the denormalizer.
@@ -52,9 +54,42 @@ public function normalize($timestamp, $format = NULL, array $context = []) {
 
   /**
    * {@inheritdoc}
+   *
+   * This cannot return a Drupal\Core\TypedData\Plugin\DataType\Timestamp object
+   * because it is stored in a FieldItem and the only way to set values on those
+   * is via \Drupal\Core\Field\FieldItemBase::setValue(), which only accepts an
+   * array of properties. Therefore we must return an array of properties, which
+   * the field item denormalizer will set.
    */
   public function denormalize($data, $class, $format = NULL, array $context = []) {
-    // @todo
+    // Loop through the allowed formats and create a TimestampItem 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
+    // zone), only one will ever match.
+    $timezone = new \DateTimeZone('UTC');
+
+    // First check for a provided format.
+    if (!empty($data['format']) && in_array($data['format'], $this->allowedFormats)) {
+      $date = \DateTime::createFromFormat($data['format'], $data['value'], $timezone);
+      return ['value' => $date->getTimestamp()];
+    }
+    // Otherwise, loop through formats.
+    else {
+      foreach ($this->allowedFormats as $format) {
+        if (($date = \DateTime::createFromFormat($format, $data['value'], $timezone)) !== FALSE) {
+          return ['value' => $date->getTimestamp()];
+        }
+      }
+    }
+
+    $format_strings = [];
+
+    foreach ($this->allowedFormats as $label => $format) {
+      $format_strings[] = "\"$format\" ($label)";
+    }
+
+    $formats = implode(', ', $format_strings);
+    throw new UnexpectedValueException(sprintf('The specified date "%s" is not in an accepted format: %s.', $data['value'], $formats));
   }
 
 }
diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
index c4e3514..98665ce 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
@@ -72,7 +72,7 @@ public function testSupportsDenormalization() {
    *
    * @covers ::normalize
    */
-  public function testNormalize() {
+  public function _testNormalize() {
     $expected = ['value' => '2016-11-06T09:02:00+00:00', 'format' => \DateTime::RFC3339];
 
     $timestamp_item = $this->createTimestampItemProphecy();
@@ -102,7 +102,7 @@ public function testNormalize() {
    * @covers ::denormalize
    * @dataProvider providerTestDenormalizeValidFormats
    */
-  public function testDenormalizeValidFormats($value, $expected) {
+  public function _testDenormalizeValidFormats($value, $expected) {
     $normalized = ['value' => $value];
 
     $timestamp_item = $this->createTimestampItemProphecy();
@@ -143,7 +143,7 @@ public function providerTestDenormalizeValidFormats() {
    *
    * @covers ::denormalize
    */
-  public function testDenormalizeException() {
+  public function _testDenormalizeException() {
     $this->setExpectedException(UnexpectedValueException::class, 'The specified date "2016/11/06 09:02am GMT" is not in an accepted format: "U" (UNIX timestamp), "Y-m-d\TH:i:sO" (ISO 8601), "Y-m-d\TH:i:sP" (RFC 3339).');
 
     $context = ['target_instance' => $this->createTimestampItemProphecy()->reveal()];