/**
 * Convert a Unix timestamp to a plain time string in the format "HH:mm:ss".
 *
 * - Converts to PlainTime using the specified or system timezone.
 * - Validates epoch unit ("seconds" | "milliseconds").
 * - Returns "" for invalid input.
 *
 * @param unix Unix timestamp (number)
 * @param options optional: epochUnit ("seconds" | "milliseconds"), timeZone (IANA)
 * @returns plain time string in "HH:mm:ss" format or "" on invalid input
 *
 * @example convertUnixToPlainTime(1706659200000) // "00:00:00"
 * @example convertUnixToPlainTime(1706659200, { epochUnit: "seconds" }) // "00:00:00"
 * @example convertUnixToPlainTime(-1) // "23:59:59.999"
 */
export declare function convertUnixToPlainTime(unix: number, options?: {
    epochUnit?: "seconds" | "milliseconds";
    timeZone?: string;
}): string;