JSFiddle - React, Tailwind, and code Playground

HTML

<div id="clock"></div>

JavaScript

window.onload = function startTime(event) {
    var today = new Date();
    var h = today.getHours();
    var m = today.getMinutes();
    var s = today.getSeconds();
    h = checkTime(h);
    m = checkTime(m);
    s = checkTime(s);
    document.getElementById('clock').innerHTML =
    h + ":" + m + ":" + s;
    var t = setTimeout(startTime, 500);
}

function checkTime(i) {
    if (i < 10) {i = "0" + i};
    return i;
}