/** Excel epoch identifier. */
export type ExcelEpoch = 'windows' | 'mac';
/** 1899-12-30 (UTC) — the Windows / 1900-system epoch in ms. */
export declare const WINDOWS_EPOCH_MS: number;
/** 1904-01-01 (UTC) — the Mac / 1904-system epoch in ms. */
export declare const MAC_EPOCH_MS: number;
/**
 * Convert an Excel serial date into a JS `Date` (UTC). The fractional part is
 * treated as a fraction of a day. For Windows 1900 the leap-bug compensation
 * kicks in for serials in [0, 60).
 */
export declare function excelToDate(serial: number, opts?: {
    epoch?: ExcelEpoch;
}): Date;
/**
 * Convert a JS `Date` into an Excel serial. The Date is read in UTC. On Windows
 * 1900, dates ≤ 1900-02-28 get a -1 day correction to account for Excel's
 * phantom leap day.
 */
export declare function dateToExcel(date: Date, opts?: {
    epoch?: ExcelEpoch;
}): number;
/** Excel duration serial (fraction of a day) → milliseconds. */
export declare function excelToDuration(serial: number): number;
/** Milliseconds → Excel duration serial (fraction of a day). */
export declare function durationToExcel(ms: number): number;
/**
 * Parse an ISO-8601 / W3CDTF datetime string into a `Date`. Same grammar as
 * `new Date(string)`; the wrapper just adds typed error reporting and a
 * stricter "must be a recognised ISO" guard.
 */
export declare function fromIso8601(s: string): Date;
/**
 * Format a `Date` as ISO-8601 with second precision in UTC. Trims the
 * millisecond fragment that `Date.toISOString()` always produces, so the output
 * matches Excel / openpyxl's W3CDTF style.
 */
export declare function toIso8601(d: Date): string;