class DateTimeItem

class DateTimeItem extends FieldItemBase implements DateTimeItemInterface {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultStorageSettings() {
        return [
            'datetime_type' => 'datetime',
        ] + parent::defaultStorageSettings();
    }
    
    /**
     * Value for the 'datetime_type' setting: store only a date.
     */
    const DATETIME_TYPE_DATE = 'date';
    
    /**
     * Value for the 'datetime_type' setting: store a date and time.
     */
    const DATETIME_TYPE_DATETIME = 'datetime';
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties['value'] = DataDefinition::create('datetime_iso8601')->setLabel(new TranslatableMarkup('Date value'))
            ->setRequired(TRUE);
        $properties['date'] = DataDefinition::create('any')->setLabel(new TranslatableMarkup('Computed date'))
            ->setDescription(new TranslatableMarkup('The computed DateTime object.'))
            ->setComputed(TRUE)
            ->setClass('\\Drupal\\datetime\\DateTimeComputed')
            ->setSetting('date source', 'value');
        return $properties;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        return [
            'columns' => [
                'value' => [
                    'description' => 'The date value.',
                    'type' => 'varchar',
                    'length' => 20,
                ],
            ],
            'indexes' => [
                'value' => [
                    'value',
                ],
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
        $element = [];
        $element['datetime_type'] = [
            '#type' => 'select',
            '#title' => $this->t('Date type'),
            '#description' => $this->t('Choose the type of date to create.'),
            '#default_value' => $this->getSetting('datetime_type'),
            '#options' => [
                static::DATETIME_TYPE_DATETIME => $this->t('Date and time'),
                static::DATETIME_TYPE_DATE => $this->t('Date only'),
            ],
            '#disabled' => $has_data,
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
        $type = $field_definition->getSetting('datetime_type');
        // Just pick a date in the past year. No guidance is provided by this Field
        // type.
        $timestamp = REQUEST_TIME - mt_rand(0, 86400 * 365);
        if ($type == DateTimeItem::DATETIME_TYPE_DATE) {
            $values['value'] = gmdate(static::DATE_STORAGE_FORMAT, $timestamp);
        }
        else {
            $values['value'] = gmdate(static::DATETIME_STORAGE_FORMAT, $timestamp);
        }
        return $values;
    }
    
    /**
     * {@inheritdoc}
     */
    public function isEmpty() {
        $value = $this->get('value')
            ->getValue();
        return $value === NULL || $value === '';
    }
    
    /**
     * {@inheritdoc}
     */
    public function onChange($property_name, $notify = TRUE) {
        // Enforce that the computed date is recalculated.
        if ($property_name == 'value') {
            $this->date = NULL;
        }
        parent::onChange($property_name, $notify);
    }

}