script.js

var timer = 0;
var timerInterval;
var ms = document.getElementById('milliseconds');
var second = document.getElementById('mainsecond');
var minute = document.getElementById('mainminute');
function start(){
  stop();
  timerInterval = setInterval( function(){
     timer += 1/60;
    msVal = Math.floor((timer - Math.floor(timer) )*100);
     secondVal = Math.floor(timer) - Math.floor(timer/60) *60;
    minuteVal = Math.floor(timer/60);
    ms.textContent = msVal <10 ? "0" + msVal.toString() : msVal;
     second.textContent = secondVal < 10 ? "0" + secondVal.toString() : secondVal;
     minute.textContent = minuteVal < 10 ? "0" + minuteVal.toString() : minuteVal;
  }, 1000/60);
 
}
function stop(){
  clearInterval(timerInterval);
}
function reset(){
    stop();
    timer = 0;
    ms.textContent = '00';
    second.textContent = '00';
    minute.textContent = '00';
  }