JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.10.3/moment.js"></script>
<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 displayTime() {
    el.innerHTML = moment(d).format("HH:mm:ss");
}

window.onClick = function() {
    d = moment(d).add(30, "minutes").toDate();
    displayTime();
}