class DateTimeWidgetBase

class DateTimeWidgetBase extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $element['value'] = [
            '#type' => 'datetime',
            '#default_value' => NULL,
            '#date_increment' => 1,
            '#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
        // storage timezone and format.
        $datetime_type = $this->getFieldSetting('datetime_type');
        if ($datetime_type === DateTimeItem::DATETIME_TYPE_DATE) {
            $storage_format = DateTimeItemInterface::DATE_STORAGE_FORMAT;
        }
        else {
            $storage_format = DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
        }
        $storage_timezone = new \DateTimezone(DateTimeItemInterface::STORAGE_TIMEZONE);
        foreach ($values as &$item) {
            if (!empty($item['value']) && $item['value'] instanceof DrupalDateTime) {
                
                /** @var \Drupal\Core\Datetime\DrupalDateTime $date */
                $date = $item['value'];
                // Adjust the date for storage.
                $item['value'] = $date->setTimezone($storage_timezone)
                    ->format($storage_format);
            }
        }
        return $values;
    }
    
    /**
     * Creates a date object for use as a default value.
     *
     * This will take a default value, apply the proper timezone for display in
     * a widget, and set the default time for date-only fields.
     *
     * @param \Drupal\Core\Datetime\DrupalDateTime $date
     *   The UTC default date.
     * @param string $timezone
     *   The timezone to apply.
     *
     * @return \Drupal\Core\Datetime\DrupalDateTime
     *   A date object for use as a default value in a field widget.
     */
    protected function createDefaultValue($date, $timezone) {
        // The date was created and verified during field_load(), so it is safe to
        // use without further inspection.
        $year = $date->format('Y');
        $month = $date->format('m');
        $day = $date->format('d');
        $date->setTimezone(new \DateTimeZone($timezone));
        if ($this->getFieldSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {
            $date->setDefaultDateTime();
            // Reset the date to handle cases where the UTC offset is greater than
            // 12 hours.
            $date->setDate($year, $month, $day);
        }
        return $date;
    }

}