/**
 * Return true if `value` is a valid ISO PlainDate string.
 *
 * - Uses regex to check format before parsing.
 * - Rejects leap seconds (e.g., "2024-12-31T23:59:60").
 * - Rejects invalid dates (e.g., "2024-02-30").
 *
 * @param value ISO PlainDate string
 * @returns boolean indicating validity
 *
 * @example isValidDate("2024-03-10") // true
 * @example isValidDate("2024-02-30") // false
 * @example isValidDate("invalid") // false
 * @example isValidDate("2024-12-31T23:59:60") // false (leap second - not a valid date)
 */
export declare function isValidDate(value: string): boolean;