/**
 * Convert a UTC datetime string to a plain time string in the format "HH:mm:ss".
 *
 * - Converts to specified timezone before extracting time.
 * - Defaults to UTC if no timezone specified.
 * - Returns "" for invalid input.
 *
 * @param value UTC datetime string (e.g., "2024-02-29T00:00:00Z")
 * @param options optional: timeZone (IANA)
 * @returns plain time string in "HH:mm:ss" format or "" on invalid input
 *
 * @example convertUtcToPlainTime("2024-02-29T00:00:00Z") // "00:00:00"
 * @example convertUtcToPlainTime("2024-02-29T00:00:00Z", { timeZone: "America/New_York" }) // "19:00:00"
 * @example convertUtcToPlainTime("invalid") // ""
 */
export declare function convertUtcToPlainTime(value: string, options?: {
    timeZone?: string;
}): string;