/**
 * Return true when the given PlainDate falls in a leap year.
 *
 * - A leap year is divisible by 4, except for century years which must be divisible by 400.
 * - Returns false for invalid input strings.
 *
 * @param value ISO PlainDate string
 * @returns boolean indicating whether the date is in a leap year
 *
 * @example isLeapYear("2024-03-15") // true
 * @example isLeapYear("2023-03-15") // false
 * @example isLeapYear("invalid") // false
 */
export declare function isLeapYear(value: string): boolean;