[рони]

ivanWantsToKnow,

<h2 class='stopwatch'>0</h2>
<button class='btn-toggle'>Start</button>

<script>
var i = 0,
    timer,
    n = 1;
document.querySelector('.btn-toggle').addEventListener('click', start, false);

function start() {

    document.querySelector('.btn-toggle').innerHTML = n ? (timer = setInterval(increment, 1), 'Pause') : (stop(), 'Start');
    n ^= 1;
}

function increment() {
    i++;
    document.querySelector('.stopwatch').innerHTML = i / 1000;
}

function stop() {
    clearInterval(timer);
}
</script>