本代码会根据2020-06-01与当前时间相差天数选取对应的图片
图片文件目录如下


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<!DOCTYPE html>
<html>

<head>
    <meta http-equiv="content-type" content="text/html;charset=utf-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
    <title>title</title>
    <style>
        #web_bg {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            min-width: 1500px;
            z-index: -10;
            zoom: 1;
            background-color: #fff;
            background-repeat: no-repeat;
            background-size: cover;
            -webkit-background-size: cover;
            -o-background-size: cover;
            background-position: center 0;
        }
    </style>
</head>

<body>
    <!--背景图片-->
    <div id="web_bg"></div>
    <!--其他代码 ... -->
</body>
<script>
    // 提前加载缓存，防止闪屏
    for (let index = 1; index < 60; index++) {
        let img = new Image();
        img.src = "imgs/" + index + ".png";
        img.onload = function () {
            document.getElementById("web_bg").style = "background-image: url(" + img.src + ")"

        }
    }

    function changeImg() {
        var i = 0;
        var end = new Date('2020-06-01');
        console.log(end)
        var start = new Date()
        interval = (end - start) / (24 * 3600 * 1000)
        console.log(parseInt(interval))
        var i = parseInt(interval) + 1
        let img = new Image();
        img.src = "imgs/" + i + ".png";
        img.onload = function () {
            document.getElementById("web_bg").style = "background-image: url(" + img.src + ")"
        }
    }


    // 一分钟刷新一次
    setInterval("changeImg()", 1000 * 60);
</script>

</html>