javascript做的一个根据table中某个td的值为日期时的倒计时

<script>
        window.onload = window.onload = function () { getTdValue(); }
        //根据传过来的天数（day）和日期（datatime），返回在传入日期上加上天数后的日期。如：day=2、datatime=2018/3/7 11:28:09，那么返回的日期就是2018/3/9 11:28:09
        function showTime(day, datatime) { 
            var dayTime = day * 24 * 60 * 60 * 1000; //参数天数的时间戳
            var nowTime = new Date(datatime).getTime(); //当天的时间戳
            var t = new Date(nowTime + dayTime).toString("yyyy/MM/dd HH:mm:ss");  //把两个时间戳转换成普通时间
            return t;
        }
        function leftTimer(enddate) {
            var leftTime = (new Date(enddate)) - new Date(); //计算剩余的毫秒数
            var days = parseInt(leftTime / 1000 / 60 / 60 / 24, 10); //计算剩余的天数
            var hours = parseInt(leftTime / 1000 / 60 / 60 % 24, 10); //计算剩余的小时
            var minutes = parseInt(leftTime / 1000 / 60 % 60, 10);//计算剩余的分钟
            var seconds = parseInt(leftTime / 1000 % 60, 10);//计算剩余的秒数
            days = checkTime(days);
            hours = checkTime(hours);
            minutes = checkTime(minutes);
            seconds = checkTime(seconds);
            if (days > 0 && hours >= 0 && minutes >= 0 && seconds >= 0)
                return days + "天" + hours + "小时" + minutes + "分" + seconds + "秒";
            else if (days == 0 && hours >= 0 && minutes >= 0 && seconds >= 0)
                return hours + "小时" + minutes + "分" + seconds + "秒";
            else if (days == 0 && hours == 0 && minutes >= 0 && seconds >= 0)
                return minutes + "分" + seconds + "秒";
            else if (days <= 0 && hours <= 0 && minutes <= 0 && seconds <= 0) {
                return "";
            }
        }
        function checkTime(i) { //将0-9的数字前面加上0，例1变为01
            if (i < 10) {
                i = "0" + i;
            }
            return i;
        }
        function getTdValue() {
            var tableId = document.getElementById("tab");
            var str = "";
            //循环获取table
            for (var i = 1; i < tableId.rows.length; i++) {
                //tableId.rows[i].cells[4].innerHTML 获取table中第5个td的值
                var date1 = new Date(), data2 = new Date(showTime(2, tableId.rows[i].cells[4].innerHTML));
                if (data2 < date1) continue;//设置的时间小于现在时间退出
                tableId.rows[i].cells[5].innerHTML = leftTimer(showTime(2, tableId.rows[i].cells[4].innerHTML))
                tableId.rows[i].cells[5].style.color = "red";//改变td颜色
                setInterval("getTdValue()", 1000);//每秒刷新一次
            }

        }
    </script>

<table style="width: 100%; height: 100%;" id="tab" border="1" cellpadding="1" cellspacing="1">
        <tr style="background-color: #CCC;">
            <th>学号</th>
            <th>姓名</th>
            <th>性别</th>
            <th>年龄</th>
            <th>入学时间</th>
            <th>倒计时</th>
        </tr>
        <tr>
            <td>1</td>
            <td>李斯</td>
            <td>男</td>
            <td>22</td>
            <td>2018/2/8 11:28:09</td>
            <td>已入学</td>
        </tr>
        <tr>
            <td>2</td>
            <td>王倩</td>
            <td>女</td>
            <td>20</td>
            <td>2018/3/7 11:28:09</td>
                 <td>已入学</td>
        </tr>
        <tr>
            <td>3</td>
            <td>于谦</td>
            <td>男</td>
            <td>18</td>
            <td>2018/3/8 13:28:09</td>
                 <td>已入学</td>
        </tr>
        <tr>
            <td>4</td>
            <td>柳浪</td>
            <td>女</td>
            <td>19</td>
            <td>2018/3/8 11:40:20</td>
                <td>已入学</td>
        </tr>
        <tr>
            <td>5</td>
            <td>诸葛亮</td>
            <td>男</td>
            <td>20</td>
            <td>2018/3/8 12:30:39</td>
                <td>已入学</td>
        </tr>
        <tr>
            <td>6</td>
            <td>东方云</td>
            <td>女</td>
            <td>21</td>
            <td>2018/3/8 11:45:20</td>
              <td>已入学</td>
        </tr>
        <tr>
            <td>7</td>
            <td>公孙策</td>
            <td>男</td>
            <td>22</td>
            <td>2018/3/8 11:50:59</td>
             <td>已入学</td>
        </tr>
        <tr>
            <td>8</td>
            <td>宝清</td>
            <td>女</td>
            <td>23</td>
            <td>2018/3/8 13:28:47</td>
               <td>已入学</td>
        </tr>
        <tr>
            <td>9</td>
            <td>智育</td>
            <td>男</td>
            <td>20</td>
            <td>2018/3/8 13:08:09</td>
                <td>已入学</td>
        </tr>
        <tr>
            <td>10</td>
            <td>柳丝丝</td>
            <td>女</td>
            <td>21</td>
            <td>2018/3/8 14:28:09</td>
               <td>已入学</td>
        </tr>
    </table>