var time = 0;

var running = 0;

function startPause() {

    if(running === 0){

        running = 1;

        increment();

        document.getElementById('startPause').innerHTML = 'PAUSE';

    }else {

        running = 0;

        document.getElementById('startPause').innerHTML = 'RESUME';

    }

}

​

function reset() {

    time = 0;

    document.getElementById('startPause').innerHTML = 'START';

    document.getElementById('output').innerHTML = '0:00:00:00';

}

​

function increment() {

    if (running == 1) {

        setTimeout(function(){

            time++;

            var mins = Math.floor(time/10/60);

            var secs = Math.floor(time/10 % 60);

            var hours = Math.floor(time/10/60/60);

            var tenths = time % 10;

            if (mins < 10) {

                mins = '0' + mins;

            }

            if (secs < 10) {

                secs = '0'+ secs;            

            }

            document.getElementById('output').innerHTML = hours + ':' + mins + ':' + secs+ ':'+tenths+ '0';

            increment();

        }, 100);

    }

}