How do I get the number of days from 2 user inputs and then multiply it with another user input which is amount in JavaScript?

Question

I'm trying to create a simple fine calculator, I have two user input dates and a user input amount. I need to subtract the 2 dates and then multiply the number of days by the fine amount. I went through a bunch of videos about dates, but none of them ever take user input. I am very new to javascript so I don't know why it won't show me my result. Could someone tell me what's wrong with my code?

if (isset($_POST['calcDiff'])) {
  $("#bdate").datetimepicker({
    timepicker: false,
    format: 'y-m-d'


  });

  $("#rdate").datetimepicker({

    timepicker: false,
    format: 'y-m-d'
  });

  function calcDiff() {

    var bdate = new Date($("#bdate").val());
    var rdate = new Date($("#rdate").val());
    var amount = $('#amount').val();

    var timeDifference = rdate.getTime() - bdate.getTime();
    var milliSecondsInOneSecond = 1000;
    var secondInOneHour = 3600;
    var hourInOneDay = 24;

    var daysDiff = timeDifference / (milliSecondsInOneSecond * secondInOneHour * hourInOneDay);
    var fine = daysDiff * amount.val();
    alert(fine);
  }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form name="finecalc" action="" method="post">
  <p>
    Borrowed date
    <input type="date" name="bdate" id="bdate" required="required" value="<?php echo $bdate; ?>" />
  </p>
  <p>
    Return date</b>
    <input type="date" name="rdate" id="rdate" required="required" value="<?php echo $rdate; ?>" /> <b>
    </p>
     <p>Enter fine amount per day</b>
    <input type="text" name="amount" size="10"><b>
       </p><button onclick="calcDiff()">Calculate Fine</button><p id="display"></p>
</form>

Answer 1

You can get the value from each input, then actually just subtract the dates to get the epoch difference and with that number you can convert it to days and multiply by the fine.

I added a JS that does just that.

function calcDiff(){
  const date1 = new Date(document.querySelector('#bdate').value);
  const date2 = new Date(document.querySelector('#rdate').value);
  const penalty = document.querySelector('input[name="amount"]').value;
  const diff_ms = date2 - date1;
 // ms -> s -> min -> h -> days
  const days = diff_ms / 1000 / 60 / 60 / 24; 
  const amount_to_pay = days * penalty;
  console.log('$' + amount_to_pay);
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form name="finecalc" action="" method="post">
  <p>
    Borrowed date
    <input type="date" name="bdate" id="bdate" required="required" value="<?php echo $bdate; ?>" />
  </p>
  <p>
    Return date</b>
    <input type="date" name="rdate" id="rdate" required="required" value="<?php echo $rdate; ?>" /> <b>
    </p>
     <p>Enter fine amount per day</b>
    <input type="text" name="amount" size="10"><b>
       </p><button type="button" onclick="calcDiff()">Calculate Fine</button><p id="display"></p>
</form>