.../EntityResource/EntityTest/EntityTestDatetimeTest.php       | 10 ++++++++++
 .../EntityResource/EntityTest/EntityTestDateRangeTest.php      | 10 ++++++++++
 2 files changed, 20 insertions(+)

diff --git a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
index a37f20d..143475b 100644
--- a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
+++ b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
@@ -148,6 +148,16 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
       $response = $this->request($method, $url, $request_options);
       $message = "The specified date \"$value\" is not in an accepted format: \"Y-m-d\\TH:i:sP\" (RFC 3339), \"Y-m-d\\TH:i:sO\" (ISO 8601).";
       $this->assertResourceErrorResponse(422, $message, $response);
+
+      // DX: 422 when date value is invalid.
+      $normalization = $this->getNormalizedPostEntity();
+      $value = '2017-13-55T20:02:00+00:00';
+      $normalization[static::$fieldName][0]['value'] = $value;
+
+      $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
+      $response = $this->request($method, $url, $request_options);
+      $message = "The specified date \"$value\" is not in an accepted format: \"Y-m-d\\TH:i:sP\" (RFC 3339), \"Y-m-d\\TH:i:sO\" (ISO 8601).";
+      $this->assertResourceErrorResponse(422, $message, $response);
     }
   }
 
diff --git a/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php b/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
index 926b7e8..e83ca38 100644
--- a/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
+++ b/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
@@ -147,6 +147,16 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
       $message = "Unprocessable Entity: validation failed.\n{$fieldName}.0.end_value: This value should be of the correct primitive type.\n";
       $this->assertResourceErrorResponse(422, $message, $response);
 
+      // DX: 422 when end date value is invalid.
+      $normalization = $this->getNormalizedPostEntity();
+      $value = '2017-13-55T20:02:00+00:00';
+      $normalization[static::$fieldName][0]['end_value'] = $value;
+
+      $request_options[RequestOptions::BODY] = $this->serializer->encode($normalization, static::$format);
+      $response = $this->request($method, $url, $request_options);
+      $message = "The specified date \"$value\" is not in an accepted format: \"Y-m-d\\TH:i:sP\" (RFC 3339), \"Y-m-d\\TH:i:sO\" (ISO 8601).";
+      $this->assertResourceErrorResponse(422, $message, $response);
+
       // @todo Expand in https://www.drupal.org/project/drupal/issues/2847041.
     }
   }