import { type UnixUnit } from "../../unix/validate/isValidUnixUnit.js";
/**
 * Convert a UTC Instant string to a unix epoch value in milliseconds or seconds.
 *
 * - Uses Temporal.Instant.from to parse.
 * - Validates unit ("seconds" | "milliseconds").
 * - Returns null for invalid input.
 *
 * @param value UTC Instant string
 * @param unit optional unit, "seconds" or "milliseconds"
 * @returns epoch number or null on invalid
 *
 * @example convertUtcToUnix("2024-02-29T00:00:00Z") // 1709164800000
 * @example convertUtcToUnix("2024-02-29T00:00:00Z", "seconds") // 1709164800
 * @example convertUtcToUnix("invalid") // null
 */
export declare function convertUtcToUnix(value: string, ...unitInput: [unit?: UnixUnit]): number | null;