var sessionTimer = {
  // Seconds left in the user's session
  timeLimit: 300,

  // The element to contain the timer count
  timeLimitSpan: null,
  
  // The countdown timer
  timer: null,
  
  sessionTimerInit: function()
  {
    sessionTimer.timeLimitSpan = document.getElementById('timelimit');
    sessionTimer.timer = setInterval(sessionTimer.updateTimer, 1000);
  },
  
  updateTimer: function()
  {
    if (--sessionTimer.timeLimit < 0)
    {
      clearInterval(sessionTimer.timer);
      return;
    }
    
    var minutes = parseInt(sessionTimer.timeLimit / 60);
    var seconds = (sessionTimer.timeLimit % 60);
    
    sessionTimer.timeLimitSpan.innerHTML =  minutes + ':' + (seconds < 10 ? '0' : '') + seconds;
  }
};

addEvent(window, 'load', sessionTimer.sessionTimerInit);