JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div> <span id="time"></span> </div>
    
    <button id="start">Start</button>
    <button id="stop">Stop</button>
</body>

JavaScript

function startTimer(duration, display) {
    var start = Date.now(),
        diff,
        minutes,
        seconds;
    function timer() {
        // get the number of seconds that have elapsed since 
        // startTimer() was called
        diff = duration - (((Date.now() - start) / 1000) | 0);

        // does the same job as parseInt truncates the float
        minutes = (diff / 60) | 0;
        seconds = (diff % 60) | 0;

        minutes = minutes < 10 ? "0" + minutes : minutes;
        seconds = seconds < 10 ? "0" + seconds : seconds;

        display.textContent = minutes + ":" + seconds; 

        if (diff <= 0) {
            // add one second so that the count down starts at the full duration
            // example 05:00 not 04:59
            start = Date.now() + 1000;
        }
    };
    // we don't want to wait a full second before the timer starts
    timer();
    var intervalId = setInterval(timer, 1000);

    document.getElementById('stop').addEventListener('click', function () {
      clearInterval(intervalId);
    });
}

window.onload = function () {
    var count = 60 * 5,
        display = document.querySelector('#time');
        document.getElementById('start').addEventListener('click', function () { 
            startTimer(count, display);
          }); 
};