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'],
                    ]));
                }
            }
        }
    }

}