/**
 * Converts a JavaScript `Date` to an Excel date serial number.
 *
 * @private
 * @param {Date} date The date to convert.
 * @returns {number}
 */
export declare function toExcelDateSerial(date: Date): number;
/**
 * Parses an ISO 8601 date string (`'YYYY-MM-DD'`) to an Excel date serial number.
 *
 * @private
 * @param {*} value Cell value — expected to be an ISO 8601 string.
 * @returns {number|null}
 */
export declare function parseIsoStringToSerial(value: unknown): number | null;
/**
 * Parses a time string to an Excel time serial number (fractional day: 0.0–1.0).
 *
 * Supports 24-hour formats (`'HH:mm'`, `'HH:mm:ss'`) and 12-hour formats
 * (`'h:mm AM/PM'`, `'h:mm:ss AM/PM'`).
 *
 * @private
 * @param {*} value Cell value — expected to be a time string.
 * @returns {number|null}
 */
export declare function parseTimeStringToSerial(value: unknown): number | null;
/**
 * Returns the Excel `numFmt` string for a date cell (OOXML built-in format ID 14).
 *
 * @private
 * @returns {string}
 */
export declare function getDateNumFmt(): string;
/**
 * Returns the Excel `numFmt` string for a time cell.
 *
 * @private
 * @returns {string}
 */
export declare function getTimeNumFmt(): string;