oninput JavaScript Date Function Calculator

Question

I found this code online and am unable to get it to work correctly can someone please tell me where I am going wrong? The description made it sound like it will work exactly how I need it to but for some reason I cannot get it to work correctly on my end. I have been putting a bunch of alerts and cannot even get them to display. I am sorry I am very new trying to teach myself JavaScript.

Input button:

<cfinput
       type="datefield"
       name="purchasedate"
       width="130"
       required="yes"
       message="Please enter purchase date."
       value="#dateformat(now(),"mm/dd/yyyy")#" 
       oninput="calculateTotal();"
       >

JavaScript Function:

function getDatePrice(date) {
    var datePrice = 0
    var theForm = document.forms["form"];
    var purchasedate = theForm.elements["purchasedate"];
    var date = new Date(purchasedate.value);
if (Object.prototype.toString.call(date) !== '[object Date]') {
        date = new Date();
    }
var today = new Date();
    var diffMilli = today - date;
    var diffDays = diffMilli * 1000 * 60 * 60 * 24;

    if (diffDays > 30) {
        datePrice= 20;
    }
    return datePrice;
}

function calculateTotal()
{
    var titleFees =  getDatePrice(date);
    var divobj = document.getElementById('totalPrice');
    divobj.style.display='block';
    divobj.innerHTML = "Estimated Transfer Fees $"+titleFees;

}

Answer 1

today - date = some time in milliseconds, if you multiply that value you'll get an astronomic number. Instead, get the number of days:

// Utils
const el = (sel, parent = document) => parent.querySelector(sel);

// Purchase form calculator
const elDate = el(`#formPurchase [name="purchasedate"]`);
const elInfo = el(`#transferFeeInfo`);

const calcTransferFees = () => { // Don't pass any arguments
  const diffMs = elDate.value ? new Date() - new Date(elDate.valueAsDate) : 0;
  const diffDays = Math.floor(diffMs / 1000 / 60 / 60 / 24);
  const isOlderThan30Days = diffDays > 30;
  const transferFees = isOlderThan30Days ? 20 : 0;
  elInfo.textContent = `Estimated Transfer Fees ${transferFees} $`;
};

elDate.addEventListener("input", calcTransferFees);

<form id="formPurchase" name="form">
  <label>Purchase Date <input type="date" name="purchasedate" required /></label>
  <div id="transferFeeInfo"></div>
</form>