tock demonstration

HTML

<script src="https://raw.githubusercontent.com/mrchimp/tock/master/tock.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/tocktimer/1.0.12/tock.min.js"></script>
<button id="start">start timer</button>
<button id="pause">pause timer</button>
<button id="reset">reset timer</button>
<input id="clock" value="00:00.000"/>

JavaScript

// countdown determines whether the timer will be a countdown timer
// interval determines how often in milliseconds the clock will update
// callback references a function that will be called every time the timer update
var t = new Tock({
  countdown:false,
  interval:10,
  callback:updateClock
});
// first create reference to the timer and control elements
var clock = document.getElementById('clock'),
    start = document.getElementById('start'),
    pause = document.getElementById('pause'),
    reset = document.getElementById('reset');

// then add event listeners to them
start.addEventListener('click',function(){
  // start with the current time in the timer
  // by passing the current time to the start function
  var current = clock.value;
  t.start(current);
},false);

pause.addEventListener('click',function(){
  t.pause();
},false);

reset.addEventListener('click',function(){
  t.reset();
  // reset the clock display as well
  clock.value = '00:00.000';
},false);

// the callback function is created here
function updateClock(){
  clock.value = t.msToTime(t.lap());
}