class DateRangeWidgetBase

class DateRangeWidgetBase extends DateTimeWidgetBase {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);

    // Wrap all of the select elements with a fieldset.
    $element['#theme_wrappers'][] = 'fieldset';
    $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;
  }

  /**
   * {@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
    // storage timezone and format.
    $datetime_type = $this
      ->getFieldSetting('datetime_type');
    if ($datetime_type === DateRangeItem::DATETIME_TYPE_DATE) {
      $storage_format = DateTimeItemInterface::DATE_STORAGE_FORMAT;
    }
    else {
      $storage_format = DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
    }
    $storage_timezone = new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE);
    $user_timezone = new \DateTimeZone(date_default_timezone_get());
    foreach ($values as &$item) {
      if (!empty($item['value']) && $item['value'] instanceof DrupalDateTime) {

        /** @var \Drupal\Core\Datetime\DrupalDateTime $start_date */
        $start_date = $item['value'];
        if ($datetime_type === DateRangeItem::DATETIME_TYPE_ALLDAY) {

          // All day fields start at midnight on the starting date, but are
          // stored like datetime fields, so we need to adjust the time.
          // This function is called twice, so to prevent a double conversion
          // we need to explicitly set the timezone.
          $start_date
            ->setTimeZone($user_timezone)
            ->setTime(0, 0, 0);
        }

        // Adjust the date for storage.
        $item['value'] = $start_date
          ->setTimezone($storage_timezone)
          ->format($storage_format);
      }
      if (!empty($item['end_value']) && $item['end_value'] instanceof DrupalDateTime) {

        /** @var \Drupal\Core\Datetime\DrupalDateTime $end_date */
        $end_date = $item['end_value'];
        if ($datetime_type === DateRangeItem::DATETIME_TYPE_ALLDAY) {

          // All day fields start at midnight on the starting date, but are
          // stored like datetime fields, so we need to adjust the time.
          // This function is called twice, so to prevent a double conversion
          // we need to explicitly set the timezone.
          $end_date
            ->setTimeZone($user_timezone)
            ->setTime(23, 59, 59);
        }

        // Adjust the date for storage.
        $item['end_value'] = $end_date
          ->setTimezone($storage_timezone)
          ->format($storage_format);
      }
    }
    return $values;
  }

  /**
   * #element_validate callback to ensure that the start date <= the end date.
   *
   * @param array $element
   *   An associative array containing the properties and children of the
   *   generic form element.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   * @param array $complete_form
   *   The complete form structure.
   */
  public function validateStartEnd(array &$element, FormStateInterface $form_state, array &$complete_form) {
    $start_date = $element['value']['#value']['object'];
    $end_date = $element['end_value']['#value']['object'];
    if ($start_date instanceof DrupalDateTime && $end_date instanceof DrupalDateTime) {
      if ($start_date
        ->getTimestamp() !== $end_date
        ->getTimestamp()) {
        $interval = $start_date
          ->diff($end_date);
        if ($interval->invert === 1) {
          $form_state
            ->setError($element, $this
            ->t('The @title end date cannot be before the start date', [
            '@title' => $element['#title'],
          ]));
        }
      }
    }
  }

}