JSFiddle - React, Tailwind, and code Playground

HTML

<form name="dateform">
    <fieldset>
        <label for="Enter leave starting date">Enter leave starting date
            <input type="text" name="firstdate" />
        </label>
        <label for="Enter leave ending date">Enter leave ending date
            <input type="text" name="seconddate" onkeyup="return dateDiff();" />
        </label>
        <label>
             Date Difference (in days):  <input type=text name=difference>
          </label>
    </fieldset>
</form>

JavaScript

function dateDiff() {
    date1 = new Date();
    date2 = new Date();
    diff = new Date();
    date1temp = new Date(dateform.firstdate.value);
    date1.setTime(date1temp.getTime());
    date2temp = new Date(dateform.seconddate.value);
    date2.setTime(date2temp.getTime());
    diff.setTime(Math.abs(date1.getTime() - date2.getTime()));
    timediff = diff.getTime();
    days = Math.floor(timediff / (1000 * 60 * 60 * 24));
    dateform.difference.value = days;
    return false;
   }