javascript做的一个根据table中某个td的值为日期时的倒计时

JavaScript代码：

HTML代码：

<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>1td>
            <td>李斯td>
            <td>男td>
            <td>22td>
            <td>2018/2/8 11:28:09td>
            <td>已入学td>
        tr>
        <tr>
            <td>2td>
            <td>王倩td>
            <td>女td>
            <td>20td>
            <td>2018/3/7 11:28:09td>
                 <td>已入学td>
        tr>
        <tr>
            <td>3td>
            <td>于谦td>
            <td>男td>
            <td>18td>
            <td>2018/3/8 13:28:09td>
                 <td>已入学td>
        tr>
        <tr>
            <td>4td>
            <td>柳浪td>
            <td>女td>
            <td>19td>
            <td>2018/3/8 11:40:20td>
                <td>已入学td>
        tr>
        <tr>
            <td>5td>
            <td>诸葛亮td>
            <td>男td>
            <td>20td>
            <td>2018/3/8 12:30:39td>
                <td>已入学td>
        tr>
        <tr>
            <td>6td>
            <td>东方云td>
            <td>女td>
            <td>21td>
            <td>2018/3/8 11:45:20td>
              <td>已入学td>
        tr>
        <tr>
            <td>7td>
            <td>公孙策td>
            <td>男td>
            <td>22td>
            <td>2018/3/8 11:50:59td>
             <td>已入学td>
        tr>
        <tr>
            <td>8td>
            <td>宝清td>
            <td>女td>
            <td>23td>
            <td>2018/3/8 13:28:47td>
               <td>已入学td>
        tr>
        <tr>
            <td>9td>
            <td>智育td>
            <td>男td>
            <td>20td>
            <td>2018/3/8 13:08:09td>
                <td>已入学td>
        tr>
        <tr>
            <td>10td>
            <td>柳丝丝td>
            <td>女td>
            <td>21td>
            <td>2018/3/8 14:28:09td>
               <td>已入学td>
        tr>
    table>