/**
 * Return the latest (maximum) of the given UTC datetime values.
 *
 * - Filters invalid values before finding maximum.
 * - Returns null if array is empty or has no valid values.
 *
 * @param utcDateTimes Array of ISO datetime strings (e.g. "2024-03-10T12:00:00Z")
 * @returns The latest UTC datetime string, or null on invalid input
 *
 * @example maxUtc(["2024-03-10T12:00:00Z", "2024-03-15T12:00:00Z", "2024-03-12T12:00:00Z"]) // "2024-03-15T12:00:00Z"
 * @example maxUtc(["invalid", "2024-03-15T12:00:00Z"]) // "2024-03-15T12:00:00Z"
 * @example maxUtc(["invalid", "also invalid"]) // null
 * @example maxUtc([]) // null
 */
export declare function maxUtc(utcDateTimes: string[]): string | null;