SO - 18963086

HTML

<span id="countdowntimer" class="blink">1280</span>

JavaScript

var timeleft = 1280;
    var downloadTimer = setInterval(function(){
    timeleft++;
    document.getElementById("countdowntimer").textContent = timeleft;
    if(timeleft >= 1296)
        clearInterval(downloadTimer);
    },3000);