function GetDays(){
var dropdt = new Date(document.getElementById("drop_date").value);
var pickdt = new Date(document.getElementById("pick_date").value);
return parseInt((150 * (dropdt - pickdt) / (24 * 3600 * 1000)));
}
function cal(){
if(document.getElementById("drop_date")){
document.getElementById("numdays2").value=GetDays();
}
}
<input type="date" class="textbox" id="pick_date" name="pickup_date" onchange="cal()"