/**
 * Return the candidate zoned datetime nearest to `target` by temporal distance.
 *
 * - Distance is measured in total days using `Temporal.Instant` epoch milliseconds.
 * - Returns `null` if the candidates array is empty or contains no valid dates.
 * - Returns `null` if `target` is invalid.
 * - On a tie (two equidistant candidates), returns the first one in array order.
 *
 * @param target ISO ZonedDateTime string to measure distance from
 * @param candidates Array of ISO ZonedDateTime strings to choose from
 * @returns The nearest candidate zoned datetime string, or null on invalid input
 *
 * @example closestZonedTo("2024-03-15T12:00:00+00:00[UTC]", ["2024-03-01T00:00:00+00:00[UTC]", "2024-03-20T00:00:00+00:00[UTC]", "2024-03-18T00:00:00+00:00[UTC]"]) // "2024-03-18T00:00:00+00:00[UTC]"
 * @example closestZonedTo("2024-03-15T12:00:00+00:00[UTC]", ["2024-03-01T00:00:00+00:00[UTC]", "2024-03-29T00:00:00+00:00[UTC]"]) // "2024-03-29T00:00:00+00:00[UTC]"
 * @example closestZonedTo("2024-03-15T12:00:00+00:00[UTC]", []) // null
 * @example closestZonedTo("invalid", ["2024-03-01T00:00:00+00:00[UTC]"]) // null
 */
export declare function closestZonedTo(target: string, candidates: string[]): string | null;