class DateTimeFieldItemList

class DateTimeFieldItemList extends FieldItemList {

  /**
   * Defines the default value as now.
   */
  const DEFAULT_VALUE_NOW = 'now';

  /**
   * Defines the default value as relative.
   */
  const DEFAULT_VALUE_CUSTOM = 'relative';

  /**
   * {@inheritdoc}
   */
  public function defaultValuesForm(array &$form, FormStateInterface $form_state) {
    if (empty($this
      ->getFieldDefinition()
      ->getDefaultValueCallback())) {
      $default_value = $this
        ->getFieldDefinition()
        ->getDefaultValueLiteral();
      $element = [
        '#parents' => [
          'default_value_input',
        ],
        'default_date_type' => [
          '#type' => 'select',
          '#title' => $this
            ->t('Default date'),
          '#description' => $this
            ->t('Set a default value for this date.'),
          '#default_value' => $default_value[0]['default_date_type'] ?? '',
          '#options' => [
            static::DEFAULT_VALUE_NOW => $this
              ->t('Current date'),
            static::DEFAULT_VALUE_CUSTOM => $this
              ->t('Relative date'),
          ],
          '#empty_value' => '',
        ],
        'default_date' => [
          '#type' => 'textfield',
          '#title' => $this
            ->t('Relative default value'),
          '#description' => $this
            ->t("Describe a time by reference to the current day, like '+90 days' (90 days from the day the field is created) or '+1 Saturday' (the next Saturday). See <a href=\"http://php.net/manual/function.strtotime.php\">strtotime</a> for more details."),
          '#default_value' => isset($default_value[0]['default_date_type']) && $default_value[0]['default_date_type'] == static::DEFAULT_VALUE_CUSTOM ? $default_value[0]['default_date'] : '',
          '#states' => [
            'visible' => [
              ':input[id="edit-default-value-input-default-date-type"]' => [
                'value' => static::DEFAULT_VALUE_CUSTOM,
              ],
            ],
          ],
        ],
      ];
      return $element;
    }
  }

  /**
   * {@inheritdoc}
   */
  public function defaultValuesFormValidate(array $element, array &$form, FormStateInterface $form_state) {
    if ($form_state
      ->getValue([
      'default_value_input',
      'default_date_type',
    ]) == static::DEFAULT_VALUE_CUSTOM) {
      $is_strtotime = @strtotime($form_state
        ->getValue([
        'default_value_input',
        'default_date',
      ]));
      if (!$is_strtotime) {
        $form_state
          ->setErrorByName('default_value_input][default_date', $this
          ->t('The relative date value entered is invalid.'));
      }
    }
  }

  /**
   * {@inheritdoc}
   */
  public function defaultValuesFormSubmit(array $element, array &$form, FormStateInterface $form_state) {
    if ($form_state
      ->getValue([
      'default_value_input',
      'default_date_type',
    ])) {
      if ($form_state
        ->getValue([
        'default_value_input',
        'default_date_type',
      ]) == static::DEFAULT_VALUE_NOW) {
        $form_state
          ->setValueForElement($element['default_date'], static::DEFAULT_VALUE_NOW);
      }
      return [
        $form_state
          ->getValue('default_value_input'),
      ];
    }
    return [];
  }

  /**
   * {@inheritdoc}
   */
  public static function processDefaultValue($default_value, FieldableEntityInterface $entity, FieldDefinitionInterface $definition) {
    $default_value = parent::processDefaultValue($default_value, $entity, $definition);
    if (isset($default_value[0]['default_date_type'])) {
      if ($definition
        ->getSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {

        // A default date only value should be in the format used for date
        // storage but in the user's local timezone.
        $date = new DrupalDateTime($default_value[0]['default_date'], date_default_timezone_get());
        $format = DateTimeItemInterface::DATE_STORAGE_FORMAT;
      }
      else {

        // A default date+time value should be in the format and timezone used
        // for date storage.
        $date = new DrupalDateTime($default_value[0]['default_date'], DateTimeItemInterface::STORAGE_TIMEZONE);
        $format = DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
      }
      $value = $date
        ->format($format);

      // We only provide a default value for the first item, as do all fields.
      // Otherwise, there is no way to clear out unwanted values on multiple value
      // fields.
      $default_value = [
        [
          'value' => $value,
          'date' => $date,
        ],
      ];
    }
    return $default_value;
  }

}