function startTimer(duration, display) {
  var timer = duration, minutes, seconds;
  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);
}
window.onload = function () {
    var session_max_time = 1440,
        display = document.querySelector('#time');
    startTimer(session_max_time, display);
};