Issue #2672950 by Vj, ilya.no, Krzysztof Domański, elandirayan, Rijidij, amateescu, shahgm, larowlan: Notice: Undefined index: #default_value in Drupal\Core\Datetime\Element\Datetime::valueCallback() (line 103

(cherry picked from commit ea33b15)

Author: alexpott

lib/Drupal/Core/Datetime/Element/Datetime.php

@@ -96,7 +96,7 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
       ];
     }
     else {
-      $date = $element['#default_value'];
+      $date = isset($element['#default_value']) ? $element['#default_value'] : NULL;
       if ($date instanceof DrupalDateTime && !$date->hasErrors()) {
         $input = [
           'date'   => $date->format($element['#date_date_format']),

tests/Drupal/KernelTests/Core/Datetime/DatetimeElementFormTest.php

@@ -64,6 +64,15 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#date_date_callbacks' => [[$this, 'datetimecallback']],
     ];
 
+    // Element without specifying the default value.
+    $form['simple_datetime_element'] = [
+      '#type' => 'datetime',
+      '#date_date_format' => ['Y-m-d'],
+      '#date_time_format' => ['H:i:s'],
+      '#date_date_element' => 'HTML Date',
+      '#date_time_element' => 'HTML Time',
+    ];
+
     $form['submit'] = [
       '#type' => 'submit',
       '#value' => t('Submit'),