Aeliot,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<span class="timer-wrp"><span class="time_in">15</span>[<span class="time_out"></span>]</span>
<script>
setInterval(function() {
myTimer()
}, 1000)
function myTimer() {
var els = document.querySelectorAll('.timer-wrp');
for ( var i = 0; i < els.length; i++) {
var el_in = els[i].querySelector('.time_in');
var el_out = els[i].querySelector('.time_out');
var t = el_out.innerHTML||+el_in.innerHTML+1;
if (t == 0)continue;
t--;
el_out.innerHTML = t;
}
}
</script>
</body>
</html>