JSFiddle - React, Tailwind, and code Playground

HTML

<input id="due_date">

JavaScript

var invoice_date = '1/1/2015';
   var invoice_duration = 100;

   var d = new Date(invoice_date).getTime()+(invoice_duration*24*60*60*1000);
d = new Date(d);
   var dFormatted =  d.getDate() + '/' + (d.getMonth()+1) + '/' + d.getFullYear();

   $("#due_date").val(dFormatted);