JSFiddle - React, Tailwind, and code Playground

HTML

<input id="initialDate" onkeyup="checkDiff(this.value)"/>
<input id="diff"/>

JavaScript

function checkDiff (initialDate) {
        if (initialDate.length == 10){
            var dateParts = initialDate.split("/");
            var date = new Date(dateParts[0], dateParts[1]-1,dateParts[2]);
            var diff = document.getElementById('diff');
            var now = new Date();
            diff.value = (now-date);
        }
}