diff -u b/core/misc/message.js b/core/misc/message.js
--- b/core/misc/message.js
+++ b/core/misc/message.js
@@ -12,7 +12,8 @@
   var messageWrapperSelector = '[data-drupal-messages]';
 
   /**
-   * Builds a div element with the aria-live attribute and attaches it to the DOM.
+   * Builds a div element with the aria-live attribute and attaches it to the
+   * DOM.
    */
   Drupal.behaviors.drupalMessage = {
     attach: function () {
@@ -62,8 +63,8 @@
    */
   Drupal.message.add = function (message, type, context) {
     if (typeof message === 'string') {
-      // Save the text and priority into a closure variable. Multiple simultaneous
-      // announcements will be concatenated and read in sequence.
+      // Save the text and priority into a closure variable. Multiple
+      // simultaneous announcements will be concatenated and read in sequence.
       messages.push({
         context: context || 'default',
         text: message,
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/hal/src/Normalizer/TimestampItemNormalizer.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\hal\Normalizer;
+
+use Drupal\Core\Field\FieldItemInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem;
+use Drupal\serialization\Normalizer\TimeStampItemNormalizerTrait;
+
+/**
+ * Converts values for TimestampItem to and from common formats for hal.
+ */
+class TimestampItemNormalizer extends FieldItemNormalizer {
+
+  use TimeStampItemNormalizerTrait;
+
+  /**
+   * The interface or class that this Normalizer supports.
+   *
+   * @var string
+   */
+  protected $supportedInterfaceOrClass = TimestampItem::class;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function normalizedFieldValues(FieldItemInterface $field_item, $format, array $context) {
+    $normalized = parent::normalizedFieldValues($field_item, $format, $context);
+    return $this->processNormalizedValues($normalized);
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php
@@ -0,0 +1,44 @@
+<?php
+
+
+namespace Drupal\Tests\rest\Functional;
+
+/**
+ * Trait for ResourceTestBase subclasses formatting expected timestamp data.
+ */
+trait BcTimestampNormalizerUnixTestTrait {
+
+  /**
+   * Formats a UNIX timestamp.
+   *
+   * Depending on the 'bc_timestamp_normalizer_unix' setting. The return will be
+   * an RFC3339 date string or the same timestamp that was passed in.
+   *
+   * @param int $timestamp
+   *   The timestamp value to format.
+   *
+   * @return string|int
+   *   The formatted RFC3339 date string or UNIX timestamp.
+   *
+   * @see \Drupal\serialization\Normalizer\TimestampItemNormalizer
+   */
+  protected function formatExpectedTimestampItemValues($timestamp) {
+    // If the setting is enabled, just return the timestamp as-is now.
+    if ($this->config('serialization.settings')->get('bc_timestamp_normalizer_unix')) {
+      return ['value' => $timestamp];
+    }
+
+    // Otherwise, format the date string to the same that
+    // \Drupal\serialization\Normalizer\TimestampItemNormalizer will produce.
+    $date = new \DateTime();
+    $date->setTimestamp($timestamp);
+    $date->setTimezone(new \DateTimeZone('UTC'));
+
+    // Format is also added to the expected return values.
+    return [
+      'value' => $date->format(\DateTime::RFC3339),
+      'format' => \DateTime::RFC3339,
+    ];
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/serialization/src/Normalizer/TimeStampItemNormalizerTrait.php
@@ -0,0 +1,85 @@
+<?php
+
+namespace Drupal\serialization\Normalizer;
+
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * A trait for TimestampItem normalization functionality.
+ */
+trait TimeStampItemNormalizerTrait {
+
+  /**
+   * Allowed timestamps formats for the denormalizer.
+   *
+   * The denormalizer allows deserialization to timestamps from three
+   * different formats. Validation of the input data and creation of the
+   * numerical timestamp value is handled with \DateTime::createFromFormat().
+   * The list is chosen to be unambiguous and language neutral, but also common
+   * for data interchange.
+   *
+   * @var string[]
+   *
+   * @see http://php.net/manual/en/datetime.createfromformat.php
+   */
+  protected $allowedFormats = [
+    'UNIX timestamp' => 'U',
+    'ISO 8601' => \DateTime::ISO8601,
+    'RFC 3339' => \DateTime::RFC3339,
+  ];
+
+  /**
+   * Processes normalized timestamp values to add a formatted date and format.
+   *
+   * @param array $data
+   * @return array
+   */
+  protected function processNormalizedValues(array $data) {
+    // Use a RFC 3339 timestamp with the time zone set to UTC to replace the
+    // timestamp value.
+    $date = new \DateTime();
+    $date->setTimestamp($data['value']);
+    $date->setTimezone(new \DateTimeZone('UTC'));
+    $data['value'] = $date->format(\DateTime::RFC3339);
+    // 'format' is not a property on TimestampItem fields. This is present to
+    // assist consumers of this data.
+    $data['format'] = \DateTime::RFC3339;
+
+    return $data;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function constructValue($data, $context) {
+    // 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));
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/serialization/src/Normalizer/TimestampItemNormalizer.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\serialization\Normalizer;
+
+use Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem;
+use Symfony\Component\Serializer\Exception\InvalidArgumentException;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+
+/**
+ * Converts values for TimestampItem to and from common formats.
+ */
+class TimestampItemNormalizer extends FieldItemNormalizer {
+
+  use TimeStampItemNormalizerTrait;
+
+  /**
+   * The interface or class that this Normalizer supports.
+   *
+   * @var string
+   */
+  protected $supportedInterfaceOrClass = TimestampItem::class;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($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);
+  }
+
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
@@ -0,0 +1,146 @@
+<?php
+
+namespace Drupal\Tests\serialization\Unit\Normalizer;
+
+use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;
+use Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem;
+use Drupal\serialization\Normalizer\TimestampItemNormalizer;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
+use Symfony\Component\Serializer\Serializer;
+
+/**
+ * Tests that entities can be serialized to supported core formats.
+ *
+ * @group serialization
+ * @coversDefaultClass \Drupal\serialization\Normalizer\TimestampItemNormalizer
+ */
+class TimestampItemNormalizerTest extends UnitTestCase {
+
+  /**
+   * @var \Drupal\serialization\Normalizer\TimestampItemNormalizer
+   */
+  protected $normalizer;
+
+  /**
+   * The test TimestampItem.
+   *
+   * @var \Drupal\Core\Field\Plugin\Field\FieldType\TimestampItem
+   */
+  protected $item;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->normalizer = new TimestampItemNormalizer();
+  }
+
+  /**
+   * @covers ::supportsNormalization
+   */
+  public function testSupportsNormalization() {
+    $timestamp_item = $this->createTimestampItemProphecy();
+    $this->assertTrue($this->normalizer->supportsNormalization($timestamp_item->reveal()));
+
+    $entity_ref_item = $this->prophesize(EntityReferenceItem::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($entity_ref_item->reveal()));
+  }
+
+  /**
+   * @covers ::supportsDenormalization
+   */
+  public function testSupportsDenormalization() {
+    $timestamp_item = $this->createTimestampItemProphecy();
+    $this->assertTrue($this->normalizer->supportsDenormalization($timestamp_item->reveal(), TimestampItem::class));
+
+    $entity_ref_item = $this->prophesize(EntityReferenceItem::class);
+    $this->assertFalse($this->normalizer->supportsNormalization($entity_ref_item->reveal(), TimestampItem::class));
+  }
+
+  /**
+   * Tests the normalize function.
+   *
+   * @covers ::normalize
+   */
+  public function testNormalize() {
+    $expected = ['value' => '2016-11-06T09:02:00+00:00', 'format' => \DateTime::RFC3339];
+
+    $timestamp_item = $this->createTimestampItemProphecy();
+    $timestamp_item->getIterator()
+      ->willReturn(new \ArrayIterator(['value' => 1478422920]));
+
+    $serializer = new Serializer();
+    $this->normalizer->setSerializer($serializer);
+
+    $normalized = $this->normalizer->normalize($timestamp_item->reveal());
+    $this->assertSame($expected, $normalized);
+  }
+
+  /**
+   * Tests the denormalize function with good data.
+   *
+   * @covers ::denormalize
+   * @dataProvider providerTestDenormalizeValidFormats
+   */
+  public function testDenormalizeValidFormats($value, $expected) {
+    $normalized = ['value' => $value];
+
+    $timestamp_item = $this->createTimestampItemProphecy();
+    // The field item should be set with the expected timestamp.
+    $timestamp_item->setValue(['value' => $expected])
+      ->shouldBeCalled();
+
+    $context = ['target_instance' => $timestamp_item->reveal()];
+
+    $denormalized = $this->normalizer->denormalize($normalized, TimestampItem::class, NULL, $context);
+    $this->assertTrue($denormalized instanceof TimestampItem);
+  }
+
+  /**
+   * Data provider for testDenormalizeValidFormats.
+   *
+   * @return array
+   */
+  public function providerTestDenormalizeValidFormats() {
+    $expected_stamp = 1478422920;
+
+    $data = [];
+
+    $data['U'] = [$expected_stamp, $expected_stamp];
+    $data['RFC3339'] = ['2016-11-06T09:02:00+00:00', $expected_stamp];
+    $data['ISO8601'] = ['2016-11-06T09:02:00+0000', $expected_stamp];
+
+    return $data;
+  }
+
+  /**
+   * Tests the denormalize function with bad data.
+   *
+   * @covers ::denormalize
+   */
+  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()];
+
+    $normalized = ['value' => '2016/11/06 09:02am GMT'];
+    $this->normalizer->denormalize($normalized, TimestampItem::class, NULL, $context);
+  }
+
+  /**
+   * Creates a Timestamp Item prophecy.
+   *
+   * @return \Prophecy\Prophecy\ObjectProphecy
+   */
+  protected function createTimestampItemProphecy() {
+    $timestamp_item = $this->prophesize(TimestampItem::class);
+    $timestamp_item->getParent()
+      ->willReturn(TRUE);
+
+    return $timestamp_item;
+  }
+
+}