Somar dia a data selecionada

by Rapha Souza

HTML

<!DOCTYPE html>
<html>
<body>

<input type="date" class="form-control" id="dataPeriodo" value="2018-11-17" >

<div class="form-group col-md-3 input-daterange">
<label>De</label>
<input type="date" class="form-control" id="descansoDe" value="2018-11-18" onblur="validaPeriodo()">
</div>

<div class="form-group col-md-3">
<label>Para</label>
<input type="date" class="form-control" id="descansoAte" onchange="calcularDatas();" >
</div>


<div class="form-group col-md-3">
<label>Exibir</label>
<input type="text" class="form-control" id="exibir">
</div>


<script>

function calcularDatas(){


var dat1 = document.getElementById("descansoDe").value;
var date1 = new Date(dat1);
//alert("dat1"+dat1);
//alert("dat11"+date1);
var dat2 = document.getElementById("descansoAte").value;
var date2 = new Date(dat2);

var timeDiff = Math.abs(date2.getTime() - date1.getTime());
var diffDays = Math.ceil(timeDiff / (1000 * 3600 * 24) + 1 ); 

document.getElementById('exibir').value = diffDays;



}
function validaPeriodo(){

var dat1 = document.getElementById("descansoDe").value;
var dat2 = document.getElementById("dataPeriodo").value;

document.getElementById("descansoAte").value = dat1;


if(dat2 > dat1){
alert(" O período deve ser menor que a data do período.");
document.getElementById('descansoAte').readOnly = true;

}

}

</script>
</body>
</html>