document.getElementById('timer').innerHTML =
  24 + ":" + 00;
startTimer();

function startTimer() {
  var presentTime = document.getElementById('timer').innerHTML;
  var timeArray = presentTime.split(/[:]+/);
  var m = timeArray[0];
  var s = checkSecond((timeArray[1] - 1));
  if(s==59){m=m-1}
  //if(m<0){alert('timer completed')}
  
  document.getElementById('timer').innerHTML =
    m + ":" + s;
  setTimeout(startTimer, 2000);
}

function checkSecond(sec) {
  if (sec < 10 && sec >= 0) {sec = "0" + sec}; // add zero in front of numbers < 10
  if (sec < 0) {sec = "59"};
  return sec;
}