createDefaultValue example

'#date_timezone' => date_default_timezone_get(),
      '#required' => $element['#required'],
    ];

    if ($this->getFieldSetting('datetime_type') == DateTimeItem::DATETIME_TYPE_DATE) {
      // A date-only field should have no timezone conversion performed, so       // use the same timezone as for storage.       $element['value']['#date_timezone'] = DateTimeItemInterface::STORAGE_TIMEZONE;
    }

    if ($items[$delta]->date) {
      $element['value']['#default_value'] = $this->createDefaultValue($items[$delta]->date, $element['value']['#date_timezone']);
    }

    return $element;
  }

  /** * {@inheritdoc} */
  public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
    // The widget form element type has transformed the value to a     // DrupalDateTime object at this point. We need to convert it back to the

$element['#element_validate'][] = [$this, 'validateStartEnd'];
    $element['value']['#title'] = $this->t('Start date');

    $element['end_value'] = [
      '#title' => $this->t('End date'),
    ] + $element['value'];

    if ($items[$delta]->start_date) {
      /** @var \Drupal\Core\Datetime\DrupalDateTime $start_date */
      $start_date = $items[$delta]->start_date;
      $element['value']['#default_value'] = $this->createDefaultValue($start_date, $element['value']['#date_timezone']);
    }

    if ($items[$delta]->end_date) {
      /** @var \Drupal\Core\Datetime\DrupalDateTime $end_date */
      $end_date = $items[$delta]->end_date;
      $element['end_value']['#default_value'] = $this->createDefaultValue($end_date, $element['end_value']['#date_timezone']);
    }

    return $element;
  }