/**
 * Return the nanosecond (0-999) for a given ISO 8601 datetime string.
 *
 * - Returns zero-padded string for nanosecond.
 * - Returns "" for invalid input.
 *
 * @param value ISO datetime string
 * @returns Nanosecond (000-999) or "" on invalid input
 *
 * @example parseNanosecondFromDateTime("2024-03-15T14:30:45.123") // "123"
 * @example parseNanosecondFromDateTime("2024-03-15T14:30:45.000") // "000"
 * @example parseNanosecondFromDateTime("invalid") // ""
 */
export declare function parseNanosecondFromDateTime(value: string): string;