Javascript 特定日期的一页上有多个JS倒计时

我有一个从某个日期开始计时的倒计时器。

我现在正在尝试调整它，这样我就可以在一个页面上有多个计时器。我已经写了下面的代码，但它不工作。有人能看到错误吗

HTML:

您可以将.innerHTML替换为.html

<div class="countup">
    <div id="targetdate" style="display: none">Thu Jan 01 1970 00:44:39 +0000</div>
    <div id="timer"></div>
</div>
<div class="countup">
    <div id="targetdate" style="display: none">Thu Jan 01 1970 00:44:49 +0000</div>
    <div id="timer"></div>
</div>

$('.countup').each(function() {
    var targetdate = $(this).children('#targetdate').innerHTML;
    var seconds_left = new Date(targetdate).getTime();
    var timer = $(this).children('#timer').innerHTML;
    var minutes, seconds;

    seconds_left = seconds_left / 1000;

    var countdownrefesh = setInterval(function () {
        // Add one to seconds
        seconds_left = seconds_left + 1;

        // do some time calculations
        days = parseInt(seconds_left / 86400);
        seconds_left = seconds_left % 86400;

        hours = parseInt(seconds_left / 3600);
        seconds_left = seconds_left % 3600;

        minutes = parseInt(seconds_left / 60);
        seconds = parseInt(seconds_left % 60);

        // format countdown string + set tag value
        timer = hours + "h:" + minutes + "m:" + seconds + "s";  

    }, 1000);
});