/**
 * Return the candidate date nearest to `target` by calendar distance.
 *
 * - Distance is measured in whole days using `Temporal.PlainDate.compare`.
 * - 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 PlainDate string to measure distance from
 * @param candidates Array of ISO PlainDate strings to choose from
 * @returns The nearest candidate date string, or null on invalid input
 *
 * @example closestDateTo("2024-03-15", ["2024-03-01", "2024-03-20", "2024-03-18"]) // "2024-03-18"
 * @example closestDateTo("2024-03-15", ["2024-03-01", "2024-03-29"]) // "2024-03-01"
 * @example closestDateTo("2024-03-15", []) // null
 * @example closestDateTo("invalid", ["2024-03-01"]) // null
 */
export declare function closestDateTo(target: string, candidates: string[]): string | null;