var interval = null;

var time = 0;

var output = $('h1');

​

function setTimer() {

  var seconds = +($("#timer").val());

  interval = setInterval(function() {

        time++;

        output.text( time );

   }, seconds*1000);

}

​

//with jquery

$('.pause').on('click', function(e) {

  e.preventDefault();

  if(interval){

    clearInterval(interval);

  }

});

​

$('.play').on('click', function(e) {

  e.preventDefault();

  setTimer();

});

​

$('.start').click(function(){

  time = 0;

  setTimer();

});