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 timer = duration, minutes, seconds;
    var intervalId = setInterval(function () {
        minutes = parseInt(timer / 60, 10)
        seconds = parseInt(timer % 60, 10);

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

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

        if (--timer < 0) {
            timer = duration;
        }
    }, 1000);
    
      document.getElementById('stop').addEventListener('click', function () {
          clearInterval(intervalId);
      });
}

window.onload = function () {
    var count = 60 * .5,
        display = document.querySelector('#time');
       // display.textContent = "00:"+count;
      document.getElementById('start').addEventListener('click', function () { 
    		startTimer(count, display);
      });       

        
};