CreatedItem.php

<?php

namespace Drupal\Core\Field\Plugin\Field\FieldType;


/**
 * Defines the 'created' entity field type.
 *
 * @FieldType(
 *   id = "created",
 *   label = @Translation("Created"),
 *   description = @Translation("An entity field containing a UNIX timestamp of when the entity has been created."),
 *   no_ui = TRUE,
 *   default_widget = "datetime_timestamp",
 *   default_formatter = "timestamp"
 * )
 */
class CreatedItem extends TimestampItem {
    
    /**
     * {@inheritdoc}
     */
    public function applyDefaultValue($notify = TRUE) {
        parent::applyDefaultValue($notify);
        // Created fields default to the current timestamp.
        $this->setValue([
            'value' => REQUEST_TIME,
        ], $notify);
        return $this;
    }

}