JSFiddle - React, Tailwind, and code Playground

HTML

<input id="nbweeks" type='number' />
<input class='datepicker' />
<input id="input-ro" class='datepicker' readonly='readonly' />

JavaScript

var curDate = new Date(Date.parse("12/10/2012"));
var nextDate = new Date(curDate);
nextDate .setDate(curDate.getDate()+2*7);
var curDay = nextDate .getDate();
var curMonth = nextDate .getMonth() + 1; //Months are zero based
var curYear = nextDate .getFullYear();
$("#input-ro").val(curMonth + "-" + curDay + "-" + curYear);