import { type UnixUnit } from "../validate/isValidUnixUnit.js";
/**
 * Convert a unix epoch value to a UTC Instant ISO string.
 *
 * - Converts to UTC Instant using Temporal.Instant.
 * - Validates value and epoch unit ("seconds" | "milliseconds").
 * - Returns "" for invalid input.
 *
 * @param value epoch value (number)
 * @param unit optional unit, "seconds" or "milliseconds"
 * @returns UTC Instant string or "" on invalid
 *
 * @example convertUnixToUtc(1709164800000) // "2024-02-29T00:00:00Z"
 * @example convertUnixToUtc(1709164800, "seconds") // "2024-02-29T00:00:00Z"
 * @example convertUnixToUtc(-1) // "1969-12-31T23:59:59.999Z"
 */
export declare function convertUnixToUtc(value: number, ...unitInput: [unit?: UnixUnit]): string;