class TimestampDatetimeWidget

class TimestampDatetimeWidget extends WidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $default_value = isset($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'] = $element['#description'] !== '' ? $element['#description'] : $this->t('Leave blank to use the time of form submission.');
    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;
  }

}