Perhaps a simpler solution.
Code:
<html>
<head>
<title></title>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
</head>
<body>
<select id = "priority" onchange = "calcdeadline()">
<option value = 0>Select priority level...</option>
<option value = 1>1 hour</option>
<option value = 4>4 hours</option>
<option value = 24>24 hours</option>
<option value = 168>7 days</option>
<option value = 720>30 days</option>
</select>
<input type = "text" id = "deadline" size = 60 readonly>
<script type = "text/javascript">
function calcdeadline() {
var myDate=new Date();
var val = Number(document.getElementById("priority").value); // from dropdown
myDate.setHours(myDate.getHours()+val);
document.getElementById('deadline').value=myDate;
}
</script>
</body>
</html>