ExcelToJsDate.js

/**
 *
 * Convert Excel dates into JS date objects
 * exampleInput: "42090" 
 * exampleOutput: "Fri Mar 27 2015 01:00:00 GMT+0100 (CET)"
 *
 * @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 Jan 1, 1900 and Jan 1, 1970, plus 1 (Google "excel leap year bug")             
  // 2. Convert to milliseconds.
  return new Date( ( excelDate - ( 25567 + 2 ) ) * 86400 * 1000 );
}