class DateRangeFieldItemList

class DateRangeFieldItemList extends DateTimeFieldItemList {

  /**
   * {@inheritdoc}
   */
  public function defaultValuesForm(array &$form, FormStateInterface $form_state) {
    if (empty($this
      ->getFieldDefinition()
      ->getDefaultValueCallback())) {
      $default_value = $this
        ->getFieldDefinition()
        ->getDefaultValueLiteral();
      $element = parent::defaultValuesForm($form, $form_state);
      $element['default_date_type']['#title'] = $this
        ->t('Default start date');
      $element['default_date_type']['#description'] = $this
        ->t('Set a default value for the start date.');
      $element['default_end_date_type'] = [
        '#type' => 'select',
        '#title' => $this
          ->t('Default end date'),
        '#description' => $this
          ->t('Set a default value for the end date.'),
        '#default_value' => $default_value[0]['default_end_date_type'] ?? '',
        '#options' => [
          static::DEFAULT_VALUE_NOW => $this
            ->t('Current date'),
          static::DEFAULT_VALUE_CUSTOM => $this
            ->t('Relative date'),
        ],
        '#empty_value' => '',
      ];
      $element['default_end_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_end_date_type']) && $default_value[0]['default_end_date_type'] == static::DEFAULT_VALUE_CUSTOM ? $default_value[0]['default_end_date'] : '',
        '#states' => [
          'visible' => [
            ':input[id="edit-default-value-input-default-end-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', $this
          ->t('The relative start date value entered is invalid.'));
      }
    }
    if ($form_state
      ->getValue([
      'default_value_input',
      'default_end_date_type',
    ]) == static::DEFAULT_VALUE_CUSTOM) {
      $is_strtotime = @strtotime($form_state
        ->getValue([
        'default_value_input',
        'default_end_date',
      ]));
      if (!$is_strtotime) {
        $form_state
          ->setErrorByName('default_value_input][default_end_date', $this
          ->t('The relative end 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',
    ]) || $form_state
      ->getValue([
      'default_value_input',
      'default_end_date_type',
    ])) {
      if ($form_state
        ->getValue([
        'default_value_input',
        'default_date_type',
      ]) == static::DEFAULT_VALUE_NOW) {
        $form_state
          ->setValueForElement($element['default_date'], static::DEFAULT_VALUE_NOW);
      }
      if ($form_state
        ->getValue([
        'default_value_input',
        'default_end_date_type',
      ]) == static::DEFAULT_VALUE_NOW) {
        $form_state
          ->setValueForElement($element['default_end_date'], static::DEFAULT_VALUE_NOW);
      }
      return [
        $form_state
          ->getValue('default_value_input'),
      ];
    }
    return [];
  }

  /**
   * {@inheritdoc}
   */
  public static function processDefaultValue($default_value, FieldableEntityInterface $entity, FieldDefinitionInterface $definition) {

    // Explicitly call the base class so that we can get the default value
    // types.
    $default_value = FieldItemList::processDefaultValue($default_value, $entity, $definition);

    // Allow either the start or end date to have a default, but not require
    // defaults for both.
    if (!empty($default_value[0]['default_date_type']) || !empty($default_value[0]['default_end_date_type'])) {

      // A default value should be in the format and timezone used for date
      // storage. All-day ranges are stored the same as date+time ranges.  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.
      $storage_format = $definition
        ->getSetting('datetime_type') == DateRangeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
      $default_values = [
        [],
      ];
      if (!empty($default_value[0]['default_date_type'])) {
        $start_date = new DrupalDateTime($default_value[0]['default_date'], DateTimeItemInterface::STORAGE_TIMEZONE);
        $start_value = $start_date
          ->format($storage_format);
        $default_values[0]['value'] = $start_value;
        $default_values[0]['start_date'] = $start_date;
      }
      if (!empty($default_value[0]['default_end_date_type'])) {
        $end_date = new DrupalDateTime($default_value[0]['default_end_date'], DateTimeItemInterface::STORAGE_TIMEZONE);
        $end_value = $end_date
          ->format($storage_format);
        $default_values[0]['end_value'] = $end_value;
        $default_values[0]['end_date'] = $end_date;
      }
      $default_value = $default_values;
    }
    return $default_value;
  }

}