JSFiddle - React, Tailwind, and code Playground

JavaScript

var R = "010413";
//генерируем № заказа (переменная R)
var day = parseInt(R.substring(0, 2), 10) - 19;
var month = parseInt(R.substring(2, 4), 10);
var year = 2000 + parseInt(R.substring(4, 6), 10);

var tmpDate = new Date(year, month-1 /* нумерация с 0 */ , day),
    newYear = tmpDate.getFullYear(),
    newMonth = (tmpDate.getMonth()+1),
    newDate = tmpDate.getDate();

if (newMonth < 10) newMonth = "0" + newMonth;
if (newDate < 10) newDate = "0" + newDate;

zzz = '' + newYear + newMonth + newDate;
alert(zzz)