class TimestampDatetimeWidget

class TimestampDatetimeWidget extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $date_format = DateFormat::load('html_date')->getPattern();
        $time_format = DateFormat::load('html_time')->getPattern();
        $default_value = isset($items[$delta]->value) ? DrupalDateTime::createFromTimestamp($items[$delta]->value) : '';
        $element['value'] = $element + [
            '#type' => 'datetime',
            '#default_value' => $default_value,
            '#date_year_range' => '1902:2037',
        ];
        $element['value']['#description'] = $this->t('Format: %format. Leave blank to use the time of form submission.', [
            '%format' => Datetime::formatExample($date_format . ' ' . $time_format),
        ]);
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
        foreach ($values as &$item) {
            // @todo The structure is different whether access is denied or not, to
            //   be fixed in https://www.drupal.org/node/2326533.
            if (isset($item['value']) && $item['value'] instanceof DrupalDateTime) {
                $date = $item['value'];
            }
            elseif (isset($item['value']['object']) && $item['value']['object'] instanceof DrupalDateTime) {
                $date = $item['value']['object'];
            }
            else {
                $date = new DrupalDateTime();
            }
            $item['value'] = $date->getTimestamp();
        }
        return $values;
    }

}