.../BcTimestampNormalizerUnixTestTrait.php         | 47 ++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php b/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php
new file mode 100644
index 0000000000..6f04ba3c68
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/BcTimestampNormalizerUnixTestTrait.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional;
+
+@trigger_error(__NAMESPACE__ . '\BcTimestampNormalizerUnixTestTrait is deprecated in Drupal 9.0.0 and will be removed before Drupal 10.0.0. Instead of BcTimestampNormalizerUnixTestTrait::formatExpectedTimestampItemValues(123456789), use (new \DateTime())->setTimestamp(123456789)->setTimezone(new \DateTimeZone("UTC"))->format(\DateTime::RFC3339), see https://www.drupal.org/node/2859657.', E_USER_DEPRECATED);
+
+/**
+ * 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);
+    // 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 [
+      'value' => $date->format(\DateTime::RFC3339),
+      'format' => \DateTime::RFC3339,
+    ];
+  }
+
+}