Issue #2992580 by Vj, Arez, andypost, jhedstrom: Custom callbacks doesn't work

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

@@ -274,7 +274,7 @@ public static function processDatetime(&$element, FormStateInterface $form_state
       // Allows custom callbacks to alter the element.
       if (!empty($element['#date_date_callbacks'])) {
         foreach ($element['#date_date_callbacks'] as $callback) {
-          if (function_exists($callback)) {
+          if (is_callable($callback)) {
             $callback($element, $form_state, $date);
           }
         }

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

+<?php
+
+namespace Drupal\KernelTests\Core\Datetime;
+
+use Drupal\Core\Datetime\DrupalDateTime;
+use Drupal\Core\Form\FormInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests DatetimeElement functionality.
+ *
+ * @group Form
+ */
+class DatetimeElementFormTest extends KernelTestBase implements FormInterface {
+
+  /**
+   * The variable under test.
+   */
+  protected $flag;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = ['datetime', 'system'];
+
+  /**
+   * Sets up the test.
+   */
+  protected function setUp() {
+    parent::setUp();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'test_datetime_element';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function datetimecallback($date) {
+    $this->flag = 'Date time callback called.';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+
+    $form['datetime_element'] = [
+      '#title' => 'datelist test',
+      '#type' => 'datetime',
+      '#default_value' => new DrupalDateTime('2000-01-01 00:00:00'),
+      '#date_date_format' => ['Y-m-d'],
+      '#date_time_format' => ['H:i:s'],
+      '#date_date_element' => 'HTML Date',
+      '#date_time_element' => 'HTML Time',
+      '#date_increment' => 1,
+      '#date_date_callbacks' => [[$this, 'datetimecallback']],
+    ];
+
+    $form['submit'] = [
+      '#type' => 'submit',
+      '#value' => t('Submit'),
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {}
+
+  /**
+   * Form validation handler.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {}
+
+  /**
+   * Tests that default handlers are added even if custom are specified.
+   */
+  public function testDatetimeElement() {
+    $form = \Drupal::formBuilder()->getForm($this);
+    $this->render($form);
+
+    $this->assertEqual(t('Date time callback called.'), $this->flag);
+  }
+
+}