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();
            $element = [
                '#parents' => [
                    'default_value_input',
                ],
                'default_date_type' => [
                    '#type' => 'select',
                    '#title' => t('Default date'),
                    '#description' => t('Set a default value for this date.'),
                    '#default_value' => isset($default_value[0]['default_date_type']) ? $default_value[0]['default_date_type'] : '',
                    '#options' => [
                        static::DEFAULT_VALUE_NOW => t('Current date'),
                        static::DEFAULT_VALUE_CUSTOM => t('Relative date'),
                    ],
                    '#empty_value' => '',
                ],
                'default_date' => [
                    '#type' => 'textfield',
                    '#title' => t('Relative default value'),
                    '#description' => t("Describe a time by reference to the current day, like '+90 days' (90 days from the day the field is created) or '+1 Saturday' (the next Saturday). See <a href=\"http://php.net/manual/function.strtotime.php\">strtotime</a> for more details."),
                    '#default_value' => isset($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', 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;
    }

}