Javascript有效日期检查在IE8 (和Firefox)中不起作用

var now = new Date();

<!-- 引入moment.js -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.29.1/moment.min.js"></script>

// 使用moment.js进行日期检查
function isValidDate(dateString) {
    return moment(dateString, 'YYYY-MM-DD', true).isValid();
}

function isValidDate(dateString) {
    var date = new Date(dateString);
    if (isNaN(date.getTime())) {
        return false;
    }
    return date.toISOString().slice(0, 10) === dateString;
}

<!--[if IE 8]>
<script>
    // IE8特定的日期检查代码
    function isValidDate(dateString) {
        var date = new Date(dateString);
        return !isNaN(date.getTime());
    }
</script>
<![endif]-->