JSFiddle - React, Tailwind, and code Playground

HTML

<div id="clock"></div>
<button onclick="onClick()">Add 30 min</button>

JavaScript

var el = document.getElementById("clock");

var d = new Date();
d.setHours(9);
d.setMinutes(0);
d.setSeconds(0);

displayTime();

function pad(t) {
    return t < 10 ? "0" + t : t;
}

function displayTime() {
    var time = pad(d.getHours()) + ":" + pad(d.getMinutes()) + ":" + pad(d.getSeconds());
    el.innerHTML = time;
}

window.onClick = function() {
    d.setMinutes(d.getMinutes() + 30);
    displayTime();
}