/**
 * Return true if `value1` and `value2` form a valid datetime range — both parseable as
 * ISO PlainDateTime strings and `value1 <= value2`.
 *
 * - Both inputs must be ISO 8601 datetime strings (e.g. `"2024-01-01T10:00:00"`).
 * - Equal `value1 === value2` is valid when `options.allowEqual` is true.
 * - Invalid input, malformed strings, or leap-second strings return `false`.
 *
 * @param value1 first ISO PlainDateTime string
 * @param value2 second ISO PlainDateTime string
 * @param options optional allowEqual flag
 * @returns boolean indicating whether the datetime range is valid
 *
 * @example isValidDateTimeRange({ value1: "2024-01-01T10:00:00", value2: "2024-12-31T23:59:59" }) // true
 * @example isValidDateTimeRange({ value1: "2024-12-31T23:59:59", value2: "2024-01-01T10:00:00" }) // false
 * @example isValidDateTimeRange({ value1: "2024-01-01T10:00:00", value2: "2024-01-01T10:00:00", options: { allowEqual: true } }) // true
 */
export declare function isValidDateTimeRange({ value1, value2, options, }: {
    value1: string;
    value2: string;
    options?: {
        allowEqual?: boolean;
    };
}): boolean;