.../EntityTest/EntityTestDatetimeTest.php              | 18 ++++++++++++++++++
 1 file changed, 18 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 143475b..51c36bf 100644
--- a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
+++ b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
@@ -112,6 +112,24 @@ protected function getNormalizedPostEntity() {
   /**
    * {@inheritdoc}
    */
+  protected function getNormalizedPatchEntity() {
+    return parent::getNormalizedPostEntity() + [
+      static::$fieldName => [
+        [
+          // Omitting the timezone is allowed, this should result in the site's
+          // timezone being used automatically. This does not make sense, but
+          // it's how it functioned in the past, so we explicitly test this to
+          // guarantee backward compatibility. ::getNormalizedPostEntity() tests
+          // the recommended case, this tests backward compatibility.
+          'value' => static::$dateString,
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   protected function assertNormalizationEdgeCases($method, Url $url, array $request_options) {
     parent::assertNormalizationEdgeCases($method, $url, $request_options);