以下是一份可供修改的元旦倒计时HTML代码及其使用教程：

<!DOCTYPE html>

<html>
<head>
    <meta charset="utf-8"/>
    <title>元旦倒计时</title>
    <style type="text/css">
        #countdown {
            font-size: 50px;
            color: red;
            text-align: center;
            margin-top: 20%;
        }
    </style>
</head>
<body>
    <div id="countdown"></div>

    <script type="text/javascript">
        var endtime = new Date("January 1, 2024 00:00:00");
        var now = new Date();

        function countdown() {
            var leftTime = endtime - now;
            if (leftTime < 0) {
                document.getElementById("countdown").innerHTML = "倒计时结束";
                return;
            }
            var days = parseInt(leftTime / 1000 / 60 / 60 / 24);
            var hours = parseInt(leftTime / 1000 / 60 / 60 % 24);
            var minutes = parseInt(leftTime / 1000 / 60 % 60);
            var seconds = parseInt(leftTime / 1000 % 60);
            document.getElementById("countdown").innerHTML = "距离元旦还有 " + days + " 天 " + hours + " 小时 " + minutes + " 分钟 " + seconds + " 秒";
            now = new Date();
            setTimeout(countdown, 1000);
        }
        countdown();
    </script>
</body>
</html>