/**
 * Formats the duration between two dates as a human-readable string.
 * @param start - The start date
 * @param end - The end date (defaults to now if not provided)
 * @returns A formatted duration string (e.g., "1h 2m 3s", "1m 23s", "45s")
 */
export declare function formatDuration(start: Date, end?: Date): string;