/**
 * Retrieves the first date of the month for the given date.
 *
 * @example
 * firstDateOfMonth("2024-01-25") // 2024-01-01
 *
 * @param date The date to check. Defaults to the current date.
 * @returns A new Date object set to the first day of the month.
 */
export declare function firstDateOfMonth(date?: Date | string | number | null): Date;