ExcelToJsDate.js

// Convert Excel dates into JS date objects
//
// @param excelDate {Number}
// @return {Date}

function getJsDateFromExcel(excelDate) {

  // JavaScript dates can be constructed by passing milliseconds
  // since the Unix epoch (January 1, 1970) example: new Date(12312512312);

  // 1. Subtract number of days between Dec 30, 1899 and Jan 1, 1970            
  // 2. Convert to milliseconds.
  if (isNaN(excelDate)) {
    return excelDate;
  } else {
    return new Date((excelDate - 25569) * 86400 * 1000);
  }
}