JSFiddle - React, Tailwind, and code Playground

HTML

<form class="form-horizontal form-berekenen" action="huren_gegevens.html">


<input type="date" class="form-control" id="dateRent" placeholder="date" required>

<input type="date" class="form-control" id="dateEnd" placeholder="date" required>

<a href="#" onclick="javascript:cost();" id="calculate">Calculate!</a>

</form>

JavaScript

function cost() {

var oneDay = 24*60*60*1000;	//hours*minutes*seconds*milliseconds
var firstDate = new Date(document.querySelector('#dateRent').value); //Would like to input it myself
var secondDate = new Date(document.querySelector('#dateEnd').value); //Would like to input it myself

var diffDays = Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay));

var priceInDays = diffDays * 250; //diffdays must be the amount between the 2 dates
                    
alert(priceInDays); //alert the days Multi by 250

}

document.querySelector('#calculate').addEventListener('click', cost);