[ваый]

<a href="#" onclick="startTimer(0, 20); return false;">Start Timer</a>
<span id="timer"></span>

<script>
function startTimer(start, stop) {
    if (start <= stop) {
        start++;
        document.getElementById('timer').innerHTML = start;
        setTimeout(function() {
            startTimer(start, stop);
        }, 100);
    }
    else document.getElementById('timer').innerHTML = stop;
}
</script>