/**
 * Retrieves the week of the year from a given date.
 *
 * @param {Date | FixedDate | null} date - The date to retrieve the week from. Defaults to the current date.
 * @returns {number} The week of the year (1-52) as per ISO 8601.
 */
export declare function weekOfYear(date?: Date | string | number | null | undefined): number;