Source

File: wp-includes/ms-blogs.php

function wpmu_update_blogs_date() {
	$site_id = get_current_blog_id();

	update_blog_details( $site_id, array( 'last_updated' => current_time( 'mysql', true ) ) );
	/**
	 * Fires after the blog details are updated.
	 *
	 * @since MU (3.0.0)
	 *
	 * @param int $blog_id Site ID.
	 */
	do_action( 'wpmu_blog_updated', $site_id );
}