/**
 * Convert a Unix timestamp to a plain datetime string in the format "YYYY-MM-DDTHH:mm:ss".
 *
 * - Converts to PlainDateTime using the specified or system timezone.
 * - Validates epoch unit ("seconds" | "milliseconds").
 * - Returns "" for invalid input.
 *
 * @param unix Unix timestamp (number)
 * @param options optional: epochUnit ("seconds" | "milliseconds"), timeZone (IANA)
 * @returns plain datetime string in "YYYY-MM-DDTHH:mm:ss" format or "" on invalid input
 *
 * @example convertUnixToPlainDateTime(1709164800000) // "2024-02-29T00:00:00"
 * @example convertUnixToPlainDateTime(1709164800, { epochUnit: "seconds" }) // "2024-02-29T00:00:00"
 * @example convertUnixToPlainDateTime(-1) // "1969-12-31T23:59:59.999"
 */
export declare function convertUnixToPlainDateTime(unix: number, options?: {
    epochUnit?: "seconds" | "milliseconds";
    timeZone?: string;
}): string;