.../EntityTest/EntityTestDatetimeTest.php             |  6 +++---
 .../EntityTest/EntityTestDateRangeTest.php            |  2 +-
 .../src/Normalizer/DateTimeIso8601Normalizer.php      | 15 ++++++++++++++-
 .../Unit/Normalizer/DateTimeIso8601NormalizerTest.php | 19 +++++++++++++++++--
 4 files changed, 35 insertions(+), 7 deletions(-)

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 51c36bf..4c92213 100644
--- a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
+++ b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
@@ -154,7 +154,7 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
 
       $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).";
+      $message = "The specified date \"$value\" is not in an accepted format: \"Y-m-d\\TH:i:sP\" (RFC 3339), \"Y-m-d\\TH:i:sO\" (ISO 8601), \"Y-m-d\\TH:i:s\" (backward compatibility — deprecated).";
       $this->assertResourceErrorResponse(422, $message, $response);
 
       // DX: 422 when date format is incorrect.
@@ -164,7 +164,7 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
 
       $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).";
+      $message = "The specified date \"$value\" is not in an accepted format: \"Y-m-d\\TH:i:sP\" (RFC 3339), \"Y-m-d\\TH:i:sO\" (ISO 8601), \"Y-m-d\\TH:i:s\" (backward compatibility — deprecated).";
       $this->assertResourceErrorResponse(422, $message, $response);
 
       // DX: 422 when date value is invalid.
@@ -174,7 +174,7 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
 
       $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).";
+      $message = "The specified date \"$value\" is not in an accepted format: \"Y-m-d\\TH:i:sP\" (RFC 3339), \"Y-m-d\\TH:i:sO\" (ISO 8601), \"Y-m-d\\TH:i:s\" (backward compatibility — deprecated).";
       $this->assertResourceErrorResponse(422, $message, $response);
     }
   }
diff --git a/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php b/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
index e83ca38..e669a22 100644
--- a/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
+++ b/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
@@ -154,7 +154,7 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
 
       $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).";
+      $message = "The specified date \"$value\" is not in an accepted format: \"Y-m-d\\TH:i:sP\" (RFC 3339), \"Y-m-d\\TH:i:sO\" (ISO 8601), \"Y-m-d\\TH:i:s\" (backward compatibility — deprecated).";
       $this->assertResourceErrorResponse(422, $message, $response);
 
       // @todo Expand in https://www.drupal.org/project/drupal/issues/2847041.
diff --git a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
index 343a258..c8c9a6b 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
@@ -71,11 +71,24 @@ public function denormalize($data, $class, $format = NULL, array $context = [])
     }
     else {
       $context['datetime_allowed_formats'] = array_diff_key($this->allowedFormats, ['date-only' => TRUE]);
-      $datetime = parent::denormalize($data, $class, $format, $context);
+      try {
+        $datetime = parent::denormalize($data, $class, $format, $context);
+      }
+      catch (\UnexpectedValueException $e) {
+        // If denormalization didn't work using any of the actively supported
+        // formats, try again with the BC format too. Explicitly label it as
+        // being deprecated and trigger a deprecation error.
+        $using_deprecated_format = TRUE;
+        $context['datetime_allowed_formats']['backward compatibility — deprecated'] = DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
+        $datetime = parent::denormalize($data, $class, $format, $context);
+      }
       unset($context['datetime_allowed_formats']);
       if (!$datetime instanceof \DateTime) {
         return $datetime;
       }
+      if (isset($using_deprecated_format)) {
+        @trigger_error('The provided datetime string format (Y-m-d\\TH:i:s) is deprecated and will be removed before Drupal 9.0.0. Use the RFC3339 format instead (Y-m-d\\TH:i:sP).', E_USER_DEPRECATED);
+      }
       $datetime->setTimezone(new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE));
       return $datetime->format(DateTimeItemInterface::DATETIME_STORAGE_FORMAT);
     }
diff --git a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
index ec8c352..1b80b3d 100644
--- a/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
+++ b/core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php
@@ -213,6 +213,21 @@ public function providerTestDenormalizeValidFormats() {
   }
 
   /**
+   * Tests the denormalize function with the date+time deprecated format.
+   *
+   * @covers ::denormalize
+   * @group legacy
+   * @expectedDeprecation The provided datetime string format (Y-m-d\TH:i:s) is deprecated and will be removed before Drupal 9.0.0. Use the RFC3339 format instead (Y-m-d\TH:i:sP).
+   */
+  public function testDenormalizeDateAndTimeDeprecatedFormat() {
+    $normalized = '2016-11-06T08:00:00';
+
+    $field_definition = $this->prophesize(FieldDefinitionInterface::class);
+    $field_definition->getSetting('datetime_type')->willReturn(DateTimeItem::DATETIME_TYPE_DATETIME);
+    $this->normalizer->denormalize($normalized, DateTimeIso8601::class, NULL, ['field_definition' => $field_definition->reveal()]);
+  }
+
+  /**
    * Tests the denormalize function with bad data for the date-only case.
    *
    * @covers ::denormalize
@@ -233,9 +248,9 @@ public function testDenormalizeDateOnlyException() {
    * @covers ::denormalize
    */
   public function testDenormalizeDateAndTimeException() {
-    $this->setExpectedException(UnexpectedValueException::class, 'The specified date "2016-11-06T08:00:00" is not in an accepted format: "Y-m-d\TH:i:sP" (RFC 3339), "Y-m-d\TH:i:sO" (ISO 8601).');
+    $this->setExpectedException(UnexpectedValueException::class, 'The specified date "on a rainy day" is not in an accepted format: "Y-m-d\TH:i:sP" (RFC 3339), "Y-m-d\TH:i:sO" (ISO 8601), "Y-m-d\TH:i:s" (backward compatibility — deprecated).');
 
-    $normalized = '2016-11-06T08:00:00';
+    $normalized = 'on a rainy day';
 
     $field_definition = $this->prophesize(FieldDefinitionInterface::class);
     $field_definition->getSetting('datetime_type')->willReturn(DateTimeItem::DATETIME_TYPE_DATETIME);