convert from/to google sheet date serial

gsheetDate.ts

import { zonedTimeToUtc, utcToZonedTime } from "date-fns-tz";

const SheetDate = {
  origin: Date.UTC(1899, 11, 30, 0, 0, 0, 0),
  dayToMs: 24 * 60 * 60 * 1000
};
function serialToDate(d: number, sheetTimeZone: string): Date {
  return zonedTimeToUtc(
    new Date(d * SheetDate.dayToMs + SheetDate.origin),
    sheetTimeZone
  );
}
function dateToSerial(date: Date, sheetTimeZone: string) {
  const msec = utcToZonedTime(date, sheetTimeZone).getTime() - SheetDate.origin;
  return msec / SheetDate.dayToMs;
}