JQuery: Countdown

Días que faltan para un evento:
http://hilios.github.io/jQuery.countdown/

$(function() {
  $("#getting-started")
  .countdown("2017/05/31", function(event) {
    $(this).text(
      event.strftime('%D días %H:%M:%S')
    );
  });
});


Countdown de segundos:

<span id="timer"></span>
<script>
var count = 30;
var counter = setInterval(timer, 1000);

function timer() {
  count=count-1;
  if (count <= 0) {
     clearInterval(counter);
     return;
  }
  document.getElementById("timer").innerHTML = count + " secs";
}
</script>

Countdown que muestre días, horas, minutos y segundos que faltan hasta una fecha:

<p id="demo"></p>
<script>
var countDownDate = new Date("May 31, 2018 15:37:25").getTime();
var x = setInterval(function() {
  var now = new Date().getTime();
  var distance = countDownDate - now;
 
  var days = Math.floor(distance / (1000 * 60 * 60 * 24));
  var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
  var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
  var seconds = Math.floor((distance % (1000 * 60)) / 1000);

  document.getElementById("demo").innerHTML = days + "d " + hours + "h "
  + minutes + "m " + seconds + "s ";

  if (distance < 0) {
    clearInterval(x);
    document.getElementById("demo").innerHTML = "EXPIRED";
  }
}, 1000);
</script>