JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <span id="Hours">00</span>:<span id="minutes">00</span>:<span id="seconds">00</span>
</div>
<button onclick="timer()">Start</button>
<button onclick="clearTimer()">Stop</button>

JavaScript

var totalSeconds,
    timer_handle;

function setTime() {
    ++totalSeconds;
    document.getElementById("seconds").innerHTML = pad(totalSeconds % 60);
    document.getElementById("minutes").innerHTML = pad(parseInt(totalSeconds / 60, 10));
     document.getElementById("Hours").innerHTML = pad(parseInt(totalSeconds / 360, 10));
}

function pad(val) {
    var valString = val + "";
    if (valString.length < 2) {
        return "0" + valString;
    } else {
        return valString;
    }
}

function timer() {
    if (!timer_handle) {
        totalSeconds = 0;
        timer_handle = setInterval(setTime, 1000);
    }
}

function clearTimer() {
    if (timer_handle) {
        clearInterval(timer_handle);
    }
    alert(document.getElementById("minutes").innerHTML + ':' + document.getElementById("seconds").innerHTML);
}