js时时更新时间

<div id="showtime"></div>

<script type="text/javascript">
        function ymdhis() {
            var obj = new Date();
            var y = obj.getFullYear();
            var m = obj.getMonth() + 1;
            var d = obj.getDate();
            var dayOfWeek = obj.getDay();
            var h = obj.getHours();
            var i = obj.getMinutes();
            var s = obj.getSeconds();
            if (m < 10) m = "0" + m;
            if (d < 10) d = "0" + d;
            if (h < 10) h = "0" + h;
            if (i < 10) i = "0" + i;
            if (s < 10) s = "0" + s;
            var arr_week = new Array("星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六");
            var time = "现在时间是：" + y + '年' + m + '月' + d + '日' + ' ' + h + ':' + i + ':' + s + " " + arr_week[dayOfWeek];
            document.getElementById("showtime").innerHTML = time;
            setTimeout(ymdhis, 1000);
        }
        ymdhis();
    </script>