function update_limit(){
    limit = limit - 1;
    if (limit < 0){
        clearTimeout(timerID);
        var form = document.getElementById("answer-form");
        form.submit();
        return;
    }
    var seconds = limit % 60;
    var minutes = (Math.floor(limit / 60)) % 60;
    var hours = (Math.floor(limit / 3600)) % 24;
    if (seconds < 10) seconds = "0" + seconds;
    if (minutes < 10) minutes = "0" + minutes;    
    time = hours + ':' + minutes + ':' + seconds;
    limit_el.firstChild.data = time;
    timerID = setTimeout("update_limit()", 1000);
}