.../serialization/src/Normalizer/DateTimeIso8601Normalizer.php      | 6 ++++++
 core/modules/serialization/src/Normalizer/DateTimeNormalizer.php    | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
index a3ec201..343a258 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
@@ -64,12 +64,18 @@ public function denormalize($data, $class, $format = NULL, array $context = [])
       $context['datetime_allowed_formats'] = array_intersect_key($this->allowedFormats, ['date-only' => TRUE]);
       $datetime = parent::denormalize($data, $class, $format, $context);
       unset($context['datetime_allowed_formats']);
+      if (!$datetime instanceof \DateTime) {
+        return $datetime;
+      }
       return $datetime->format(DateTimeItemInterface::DATE_STORAGE_FORMAT);
     }
     else {
       $context['datetime_allowed_formats'] = array_diff_key($this->allowedFormats, ['date-only' => TRUE]);
       $datetime = parent::denormalize($data, $class, $format, $context);
       unset($context['datetime_allowed_formats']);
+      if (!$datetime instanceof \DateTime) {
+        return $datetime;
+      }
       $datetime->setTimezone(new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE));
       return $datetime->format(DateTimeItemInterface::DATETIME_STORAGE_FORMAT);
     }
diff --git a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
index a6353a0..e0a72a9 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
@@ -68,6 +68,12 @@ protected function getNormalizationTimezone() {
    * {@inheritdoc}
    */
   public function denormalize($data, $class, $format = NULL, array $context = []) {
+    // This only knows how to denormalize datetime strings and timestamps. If
+    // something else is received, let validation constraints handle this.
+    if (!is_string($data) && !is_numeric($data)) {
+      return $data;
+    }
+
     // Loop through the allowed formats and create a \DateTime 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