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>