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;
    }

}