Maxime Roux
--- a/core/tests/Drupal/KernelTests/Core/Datetime/Element/DatetimeFormElementTest.php 0 → 100644
+++ b/core/tests/Drupal/KernelTests/Core/Datetime/Element/DatetimeFormElementTest.php 0 → 100644
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp(): void {
+    parent::setUp();
+
+    $this->installEntitySchema('date_format');
+    $this->installConfig(['system']);
+    $this->formBuilder = $this->container->get('form_builder');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId(): string {
--- a/core/lib/Drupal/Core/Datetime/Element/Datetime.php
+++ b/core/lib/Drupal/Core/Datetime/Element/Datetime.php
    if ($element['#date_date_element'] != 'none') {

      $date_format = $element['#date_date_element'] != 'none' ? static::getHtml5DateFormat($element) : '';
+      // With a datetime-local input, the date value is always normalized to
+      // the format Y-m-d\TH:i
+      // @see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/datetime-local
+      // 'html_datetime' returned by static::getHtml5DateFormat($element) is not
+      // a valid format.
+      // @see https://www.drupal.org/project/drupal/issues/3505318
+      if ($element['#date_date_element'] === 'datetime-local') {
+        $date_format = DateFormat::load('html_date')->getPattern() . '\T' . DateFormat::load('html_time')->getPattern();