JSFiddle - React, Tailwind, and code Playground

JavaScript

function retornaData(){
var dataAdmissao = "\/Date(1420077600000)\/";
var milisegundos = parseInt(dataAdmissao.slice(dataAdmissao.indexOf('(') + 1, dataAdmissao.indexOf(')')));
var data = new Date(milisegundos);

    return (("0" + (data.getMonth() + 1)).slice(-2) + "/" +  ("0" + (data.getDate())).slice(-2) + "/" + data.getFullYear())
}

alert(retornaData());