diff --git a/core/modules/datetime_range/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php b/core/modules/datetime_range/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
index 5bb4213..e05895b 100644
--- a/core/modules/datetime_range/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
+++ b/core/modules/datetime_range/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Url;
 use Drupal\entity_test\Entity\EntityTest;
+use Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface;
 use Drupal\datetime_range\Plugin\Field\FieldType\DateRangeItem;
 use Drupal\field\Entity\FieldConfig;
 use Drupal\field\Entity\FieldStorageConfig;
@@ -18,4 +19,191 @@
  */
 class EntityTestDatetimeTest extends EntityTestResourceTestBase {
 
+  use AnonResourceTestTrait;
+
+  /**
+   * The starting ISO date string to use throughout the test.
+   *
+   * @var string
+   */
+  protected static $startDateString = '2017-03-01T20:02:00';
+
+  /**
+   * The ending ISO date string to use throughout the test.
+   *
+   * @var string
+   */
+  protected static $endDateString = '2017-03-02T20:02:00';
+
+  /**
+   * Datetime test field name.
+   *
+   * @var string
+   */
+  protected static $fieldName = 'field_daterange';
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['datetime', 'datetime_range', 'entity_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    // Add daterange field.
+    FieldStorageConfig::create([
+      'field_name' => static::$fieldName,
+      'type' => 'daterange',
+      'entity_type' => static::$entityTypeId,
+      'settings' => ['datetime_type' => DateRangeItem::DATETIME_TYPE_DATETIME],
+    ])
+      ->save();
+
+    FieldConfig::create([
+      'field_name' => static::$fieldName,
+      'entity_type' => static::$entityTypeId,
+      'bundle' => $this->entity->bundle(),
+//      'settings' => ['default_value' => static::$startDateString],
+    ])
+      ->save();
+
+    // Reload entity so that it has the new field.
+    $this->entity = $this->entityStorage->load($this->entity->id());
+    $this->entity->set(static::$fieldName, [
+      'value' => static::$startDateString,
+      'end_value' => static::$endDateString,
+    ]);
+    $this->entity->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function createEntity() {
+    $entity_test = EntityTest::create([
+      'name' => 'Llama',
+      'type' => static::$entityTypeId,
+      static::$fieldName => [
+        'value' => static::$startDateString,
+        'end_value' => static::$endDateString,
+      ],
+    ]);
+    $entity_test->setOwnerId(0);
+    $entity_test->save();
+
+    return $entity_test;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getExpectedNormalizedEntity() {
+    return parent::getExpectedNormalizedEntity() + [
+        static::$fieldName => [
+          [
+            'value' => $this->entity->get(static::$fieldName)->value,
+            'end_value' => $this->entity->get(static::$fieldName)->end_value,
+          ],
+        ],
+      ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getNormalizedPostEntity() {
+    return parent::getNormalizedPostEntity() + [
+        static::$fieldName => [
+          [
+            'value' => static::$startDateString,
+            'end_value' => static::$endDateString,
+          ],
+        ],
+      ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function assertNormalizationEdgeCases($method, Url $url, array $request_options) {
+    parent::assertNormalizationEdgeCases($method, $url, $request_options);
+
+    if ($this->entity->getEntityType()->hasKey('bundle')) {
+      $fieldName = static::$fieldName;
+      $fieldFormat = DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
+
+      // DX: 422 when date type is incorrect.
+
+      // Errors?
+      $value = ['2017', '03', '01', '21', '53', '00'];
+      $end_value = static::$endDateString;
+      $message = "Unprocessable Entity: validation failed.\n{$fieldName}.0: The daterange start value must be a string.\n{$fieldName}.0: The daterange end value must be a string.\n{$fieldName}.0.value: This value should be of the correct primitive type.\n{$fieldName}.0.end_value: This value should be of the correct primitive type.\n";
+      $this->doEdgeCaseCall($method, $url, $request_options, $value, $end_value, $message);
+
+      // Works!
+      $value = static::$startDateString;
+      $end_value = ['2017', '03', '02', '21', '53', '00'];
+      $message = "Unprocessable Entity: validation failed.\n{$fieldName}.0: The daterange end value must be a string.\n{$fieldName}.0.end_value: This value should be of the correct primitive type.\n";
+      $this->doEdgeCaseCall($method, $url, $request_options, $value, $end_value, $message);
+
+      // Works??
+      $value = ['2017', '03', '01', '21', '53', '00'];
+      $end_value = ['2017', '03', '02', '21', '53', '00'];
+      $message = "Unprocessable Entity: validation failed.\n{$fieldName}.0: The daterange start value must be a string.\n{$fieldName}.0: The daterange end value must be a string.\n{$fieldName}.0.value: This value should be of the correct primitive type.\n{$fieldName}.0.end_value: This value should be of the correct primitive type.\n";
+      $this->doEdgeCaseCall($method, $url, $request_options, $value, $end_value, $message);
+
+      // @todo Adjust the stuff below...
+      return;
+
+      // DX: 422 when date format is incorrect.
+      $normalization = $this->getNormalizedPostEntity();
+      $value = '2017-03-01';
+      $normalization[static::$fieldName][0]['value'] = $value;
+
+      $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
+      $response = $this->request($method, $url, $request_options);
+      $message = "Unprocessable Entity: validation failed.\n{$fieldName}.0: The daterange start value '{$value}' is invalid for the format '{$fieldFormat}'\n";
+      $this->assertResourceErrorResponse(422, $message, $response);
+
+      // DX: 422 when date format is incorrect.
+      $normalization = $this->getNormalizedPostEntity();
+      $value = '2017-13-55T20:02:00';
+      $normalization[static::$fieldName][0]['value'] = $value;
+
+      $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
+      $response = $this->request($method, $url, $request_options);
+      $message = "Unprocessable Entity: validation failed.\n{$fieldName}.0: The daterange start value '{$value}' did not parse properly for the format '{$fieldFormat}'\n{$fieldName}.0.value: This value should be of the correct primitive type.\n";
+      $this->assertResourceErrorResponse(422, $message, $response);
+    }
+  }
+
+  /**
+   * Performs a REST call to test an edge case.
+   *
+   * @param string $method
+   *   HTTP method.
+   * @param \Drupal\Core\Url $url
+   *   URL to request.
+   * @param array $request_options
+   *   Request options to apply.
+   * @var mixed $value
+   *   Foo.
+   * @var mixed $end_value
+   *   Foo.
+   * @var string $message
+   *   Foo.
+   */
+  protected function doEdgeCaseCall($method, Url $url, array $request_options, $value, $end_value, $message) {
+    $normalization = $this->getNormalizedPostEntity();
+    $normalization[static::$fieldName][0]['value'] = $value;
+    $normalization[static::$fieldName][0]['end_value'] = $end_value;
+
+    $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
+    $response = $this->request($method, $url, $request_options);
+    $this->assertResourceErrorResponse(422, $message, $response);
+  }
+
 }
diff --git a/core/modules/datetime_range/src/Plugin/Validation/Constraint/DateRangeFormatConstraintValidator.php b/core/modules/datetime_range/src/Plugin/Validation/Constraint/DateRangeFormatConstraintValidator.php
index aec6c1b..752a7d6 100644
--- a/core/modules/datetime_range/src/Plugin/Validation/Constraint/DateRangeFormatConstraintValidator.php
+++ b/core/modules/datetime_range/src/Plugin/Validation/Constraint/DateRangeFormatConstraintValidator.php
@@ -17,69 +17,77 @@ class DateRangeFormatConstraintValidator extends ConstraintValidator {
    * {@inheritdoc}
    */
   public function validate($item, Constraint $constraint) {
+    $stop = FALSE;
     /* @var $item \Drupal\datetime_range\Plugin\Field\FieldType\DateRangeItem */
     if (isset($item)) {
       $value = $item->getValue()['value'];
       $end_value = $item->getValue()['end_value'];
+
       if (!is_string($value)) {
         $this->context->addViolation($constraint->badStartType);
+        $stop = TRUE;
       }
-      elseif (!is_string($end_value)) {
+      if (!is_string($end_value)) {
         $this->context->addViolation($constraint->badEndType);
+        $stop = TRUE;
       }
-      else {
-        $datetime_type = $item->getFieldDefinition()->getSetting('datetime_type');
-        $format = $datetime_type === DateRangeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
 
-        $start_date = NULL;
-        try {
-          $start_date = DateTimePlus::createFromFormat($format, $value, new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE));
-        }
-        catch (\InvalidArgumentException $e) {
-          $this->context->addViolation($constraint->badStartFormat, [
-            '@value' => $value,
-            '@format' => $format,
-          ]);
-          return;
-        }
-        catch (\UnexpectedValueException $e) {
-          $this->context->addViolation($constraint->badStartValue, [
-            '@value' => $value,
-            '@format' => $format,
-          ]);
-          return;
-        }
-        if ($start_date === NULL || $start_date->hasErrors()) {
-          $this->context->addViolation($constraint->badStartFormat, [
-            '@value' => $value,
-            '@format' => $format,
-          ]);
-        }
+      if ($stop) return;
+
+      // @todo Reorder this w/ $stop...
+      return;
+
+      $datetime_type = $item->getFieldDefinition()->getSetting('datetime_type');
+      $format = $datetime_type === DateRangeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
 
-        $end_date = NULL;
-        try {
-          $end_date = DateTimePlus::createFromFormat($format, $end_value, new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE));
-        }
-        catch (\InvalidArgumentException $e) {
-          $this->context->addViolation($constraint->badEndFormat, [
-            '@end_value' => $end_value,
-            '@format' => $format,
-          ]);
-          return;
-        }
-        catch (\UnexpectedValueException $e) {
-          $this->context->addViolation($constraint->badEndValue, [
-            '@end_value' => $end_value,
-            '@format' => $format,
-          ]);
-          return;
-        }
-        if ($end_date === NULL || $end_date->hasErrors()) {
-          $this->context->addViolation($constraint->badEndFormat, [
-            '@end_value' => $end_value,
-            '@format' => $format,
-          ]);
-        }
+      $start_date = NULL;
+      try {
+        $start_date = DateTimePlus::createFromFormat($format, $value, new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE));
+      }
+      catch (\InvalidArgumentException $e) {
+        $this->context->addViolation($constraint->badStartFormat, [
+          '@value' => $value,
+          '@format' => $format,
+        ]);
+        return;
+      }
+      catch (\UnexpectedValueException $e) {
+        $this->context->addViolation($constraint->badStartValue, [
+          '@value' => $value,
+          '@format' => $format,
+        ]);
+        return;
+      }
+      if ($start_date === NULL || $start_date->hasErrors()) {
+        $this->context->addViolation($constraint->badStartFormat, [
+          '@value' => $value,
+          '@format' => $format,
+        ]);
+      }
+
+      $end_date = NULL;
+      try {
+        $end_date = DateTimePlus::createFromFormat($format, $end_value, new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE));
+      }
+      catch (\InvalidArgumentException $e) {
+        $this->context->addViolation($constraint->badEndFormat, [
+          '@end_value' => $end_value,
+          '@format' => $format,
+        ]);
+        return;
+      }
+      catch (\UnexpectedValueException $e) {
+        $this->context->addViolation($constraint->badEndValue, [
+          '@end_value' => $end_value,
+          '@format' => $format,
+        ]);
+        return;
+      }
+      if ($end_date === NULL || $end_date->hasErrors()) {
+        $this->context->addViolation($constraint->badEndFormat, [
+          '@end_value' => $end_value,
+          '@format' => $format,
+        ]);
       }
     }
   }