CreatedItem.php

<?php

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

use Drupal\Core\Field\Attribute\FieldType;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Defines the 'created' entity field type.
 */
class CreatedItem extends TimestampItem {
  
  /**
   * {@inheritdoc}
   */
  public function applyDefaultValue($notify = TRUE) {
    parent::applyDefaultValue($notify);
    // Created fields default to the current timestamp.
    $this->setValue([
      'value' => \Drupal::time()->getRequestTime(),
    ], $notify);
    return $this;
  }

}