excel-date-to-and-from.js

function excelDateToJsDate(excelDate) {
  // this code was originally written in https://gist.github.com/christopherscott/2782634
  return new Date(
    (excelDate - (25567 + 1)) * 86400 * 1000
  );
}

function jsDateToExcelDate(date) {
  // just working backwards from the code above
  return (date.getTime() / (86400 * 1000)) + (25567 + 1);
}

function isoStringToExcelDate(isoString) {
  return jsDateToExcelDate(new Date(isoString));
}