You can do it like this.
<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
function GetDays(){
var dropdt = new Date(document.getElementById("drop_date").value);
var pickdt = new Date(document.getElementById("pick_date").value);
return parseInt((dropdt - pickdt) / (24 * 3600 * 1000));
}
function cal(){
if(document.getElementById("drop_date")){
document.getElementById("numdays2").value=GetDays();
}
}
</script>
</head>
<body>
<div id="reserve_form">
<div id="pickup_date"><p><label class="form">Pickup Date:</label><input type="date" class="textbox" id="pick_date" name="pickup_date" onchange="cal()"</p></div>
<div id="dropoff_date"><p><label class="form">Dropoff Date:</label><input type="date" class="textbox" id="drop_date" name="dropoff_date" onchange="cal()"/></p></div>
<div id="numdays"><label class="form">Number of days:</label><input type="text" class="textbox" id="numdays2" name="numdays"/></div>
</div>
</body>
