Edit in JSFiddle

var condition = ko.observable(true); //condition to keep the timer running

var viewModel = {
    timer: new koTimer(180, {
        notifyTimeMarks: [60, 120],
        callback: function () {
            $('#timer').html('<span class="fin">Time is up!</span>');
        },
        keepGoing: condition //when the condition is false, the timer will stop
    }),
    stop: function () {
        condition(false); //set condition to false
    }
};

//bind to TimeIsUp event
$(document).on(viewModel.timer.Events.TimeIsUp, function () {
    //this event is fired when the timer reaches zero
    console.log('Time is up!');
});

//bind to TimeMarkHit event
$(document).on(viewModel.timer.Events.TimeMarkHit, function (ev) {
    //this event is fired every time a time mark is hit
    console.log('Time elapsed: ' + ev.timeElapsed + ' seconds');
    if (ev.timeElapsed === 120) {
        //during last minute, animate size of counter
        //animation time in milliseconds = 1 minute
        $('#timer').find('span').animate({ fontSize: '80px' }, 60 * 1000); 
    }
});

//apply ko bindings
ko.applyBindings(viewModel, $('.main')[0]);

<div class="main">
    <h1>THIS IS A TIMER</h1>
    <div id="timer">
        <span data-bind="text: timer.TimeLeftStr"></span>
    </div>
    <button data-bind="click: stop">Stop Timer</button>
</div>