Edit in JSFiddle

var oTimer;
var Timer = function (pSeconds, pDisplay, pCallback, pAutoStart) {
    var me = this, 
        tHandler = '',
        iTime  = 0,
        sDisplay = '',
        fCallback = '',
        iTotSteps = 0,
        iCurrStep = 0;
	/* ***** Private Methods ******************************** */
    function Step () {
        sDisplay.innerHTML = FormatTime(iTime)
        if (iCurrStep == iTotSteps) me.Stop();
        else {
            iCurrStep++;
            iTime -= 1000;
            tHandler = setTimeout(Step, 1000);
        }
    }
    function FormatTime (time) {
        time /= 1000;
        var min = Math.floor((time %= 3600) / 60),
            sec = time % 60;
        return Pad(min) + ":" + Pad(sec);
    }
    function Pad (pNum) {
       return (! pNum ? '00' : (pNum < 10 ? '0' + pNum : pNum));
    }
    function Init (pSeconds, pDisplay, pCallback, pAutoStart) {
        iTime = pSeconds * 1000;
        iTotSteps = pSeconds;
        iCurrStep = 0;
        sDisplay = document.getElementById(pDisplay);
        fCallback = pCallback;
        if (pAutoStart) me.Start();
    }
    /* ****** Public Methods ******************************** */
	// pSeconds; pDisplay: id of html element to display time left; pCallback: function to call when Stop
    this.Start = function () {
       Step();
    }
    this.Stop = function () {
      if (tHandler) clearTimeout(tHandler);
      fCallback();
    }
    //
    Init (pSeconds, pDisplay, pCallback, pAutoStart);
} // Timer

function doStart () {
   oTimer = new Timer(70, "theDisplay", doStop, 1);
   document.getElementById('message').innerHTML = '';
}

function doStop (pFromButton) {
   if (oTimer && pFromButton) oTimer.Stop();
   document.getElementById('message').innerHTML = 'TIMEOUT !!!!!!';
}

<p><button onclick="doStart()">Start</button> Time: <span id="theDisplay"></span></p>
<p><button onclick="doStop(1)">Stop</button> <span id="message"></span></p>