May be this could help
<script type="text/javascript" language="javascript">
function AddDays(toAdd) {
if (!toAdd || toAdd == '' || isNaN(toAdd)) return;
var d = new Date();
d.setDate(d.getDate() + parseInt(toAdd));
document.getElementById("result").innerHTML = d.getDate() + "/" + d.getMonth() + "/" + d.getFullYear();
}
function SubtractDays(toAdd) {
if (!toAdd || toAdd == '' || isNaN(toAdd)) return;
var d = new Date();
d.setDate(d.getDate() - parseInt(toAdd));
document.getElementById("result").innerHTML = d.getDate() + "/" + d.getMonth() + "/" + d.getFullYear();
}
</script>
---------------------- UI ---------------
<div id="result">
</div>
<input type="text" value="0" onkeyup="AddDays(this.value);" />
<input type="text" value="0" onkeyup="SubtractDays(this.value);" />