Cuenta atrás hasta una fecha javascript

[El_Astur]

Buenas Zed,

con lo que me has comentado me funciona correctamente, le he agregado un texto que aparece al final de la cuenta atrás. Mil gracias!!! Dejo el código por si le pudiera interesar a alguien:

Código:

<script tyoe="text/javascript">
// set the date we're counting down to
var target_date = new Date("Apr 20, 2016").getTime();
 
// variables for time units
var days, hours, minutes, seconds;
 
// get tag element
var countdown =  document.getElementById("countdown");
var days_span = document.createElement("SPAN");
days_span.className = 'days';
countdown.appendChild(days_span);
var hours_span = document.createElement("SPAN");
hours_span.className = 'hours';
countdown.appendChild(hours_span);
var minutes_span = document.createElement("SPAN");
minutes_span.className = 'minutes';
countdown.appendChild(minutes_span);
var secs_span = document.createElement("SPAN");
secs_span.className = 'secs';
countdown.appendChild(secs_span);
 
// update the tag with id "countdown" every 1 second
var timerId = setInterval(function () {
 
    // find the amount of "seconds" between now and target
    var current_date = new Date().getTime();
    var seconds_left = (target_date - current_date) / 1000;
 
    // do some time calculations
    days = parseInt(seconds_left / 86400);
    seconds_left = seconds_left % 86400;
     
    hours = parseInt(seconds_left / 3600);
    seconds_left = seconds_left % 3600;
     
    minutes = parseInt(seconds_left / 60);
    seconds = parseInt(seconds_left % 60);
     
    // format countdown string + set tag value.
  
    days_span.innerHTML = '<span>' + days + '</span>' + 'Days';
    hours_span.innerHTML = '<span>' + hours + '</span>' + 'Hours';
    minutes_span.innerHTML = '<span>' + minutes + '</span>' + 'Minutes';
    secs_span.innerHTML = '<span>' + seconds + '</span>' + 'Seconds';

    if(seconds <= 0 && minutes <= 0 && hours <= 0 && days <= 0){
      countdown.innerHTML = "<span class='final'>" + "finalizado" +   "</span>";
    }
    clearInterval(timerId); 
 
}, 1000);
</script>