Calculate a date from an initial date and a deadline

Question

I need that when selecting a start date and a period (in months) the script calculates in months the end date.

Detail: when clicking outside the period field already do all the calculation displaying the final date in the corresponding field.

I don’t have anything ready yet due to my lack of Javascript knowledge.

Answer 1

Maybe I got a little big the JS, but it already does what you asked.

function calcDate() {
  var dat = document.getElementById("data").value;
  var meses = document.getElementById("meses").value;
  if(dat != "" && meses != "") {
    var sp = dat.split("/");
    dat = new Date(sp[2], sp[1]-1, sp[0]);
    var m = meses%12;
    var y = Math.floor(meses/12);
    var tmp = dat.setMonth(dat.getMonth()+m);
    var tmp = dat.setYear(dat.getFullYear()+y);
    var f = new Date(tmp);
    document.getElementById("final").value = ("0" + f.getDate()).slice(-2) + "/" + ("0" + (f.getMonth() + 1)).slice(-2) + "/" + f.getFullYear();
  }
}

<input onblur="calcDate()" id="data" placeholder="dd/mm/yyyy">
<input onblur="calcDate()" id="meses" type="number" placeholder="Prazo em meses">
<input id="final" readonly placeholder="Data Final">