Timezone conversion in Javascript for Quickbooks API

quickbooks_js_timezone.js

/** Add hours to a given date object **/
Date.prototype.addHours = function(h) {    
   this.setTime(this.getTime() + (h*60*60*1000)); 
   return this;   
}

/** overall function to create QBO invoices **/
function generateInvoices(){
...
  var invoiceDate = new Date();
  invoiceDate.addHours(18); // to add 18 hours to the datetime object.
...
}