Adicionar ou Subtrair dias a data

by Rapha Souza

HTML

<script src="//code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<link rel="stylesheet" href="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
<input name="descansoDe" type="date" id="descansoDe"  onblur="calculaDataEfetivacao();"/>


<input  name="qtdDiasAviso" type="number" id="qtdDiasAviso" value="10"  />

<input name="dataAvisoEfetivacao" type="text" id="dataAvisoEfetivacao"  />




<script type="text/javascript">

function calculaDataEfetivacao() {
var datainicial = document.getElementById("descansoDe").value;
var dias = parseInt(document.getElementById("qtdDiasAviso").value);
var partes = datainicial.split("-");
var ano = partes[0];
var mes = partes[1]-1;
var dia = partes[2];

datainicial = new Date(ano,mes,dia);
datafinal = new Date(datainicial);
//trocar o + dias para = dias
datafinal.setDate(datafinal.getFullYear() + 10);

var dd = ("0" + datafinal.getDate()).slice(-2);
var mm = ("0" + (datafinal.getMonth()+1)).slice(-2);
var y =  datafinal.getFullYear();

var dataformatada = dd + '/' + mm + '/' + y;
//var dataformatada = y + '-' + mm + '-' + dd;
document.getElementById('dataAvisoEfetivacao').value = dataformatada;

}

</script>