回答

直接根据秒来判断，代码如下：

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">

    <script type="text/javascript">

        function getTimeDistance(time) {
            // 支持传入10位或13位毫秒数，如 1587367194536,"1587367194"
            // 支持传入日期格式，如 "2020/4/20 15:31:18"

            if (typeof time == "number" || Number(time) == time) {
                if (String(time).length == 10) {
                    time = Number(time) * 1000
                } else if (String(time).length == 13) {
                    time = Number(time)
                } else {
                    console.log("时间格式错误");
                    return time;
                }
            } else {
                if (typeof time == "string" && time.split(" ").length == 2 && time.split(/[- : \/]/).length == 6) {
                    time = new Date(time.replace(/\-/g, '/')).getTime();
                } else {
                    console.log("时间格式错误");
                    return time;
                }
            }

            // 处理之后的time为13位数字格式的毫秒数

            var date_now = new Date();
            var date_time = new Date(time);
            var distance = date_now.getTime() - time;

            var days = parseInt(distance / (1000 * 60 * 60 * 24));
            if (days == 1) {
                return "昨天"
            } else if (days > 1 && days < 4) {
                return days + "天前";
            } else if (days > 3) {
                // 超过3天的，返回日期，如 2018-12-05
                // 如果是今年的，就省去年份，返回 12-05
                var year = date_time.getFullYear();
                var month = date_time.getMonth() + 1;
                if (month < 10) {
                    month = "0" + month;
                }
                var day = date_time.getDate();
                if (day < 10) {
                    day = "0" + day;
                }
                if (date_now.getFullYear() == year) {
                    return month + "-" + day;
                } else {
                    return year + "-" + month + "-" + day;
                }
            }

            var hours = parseInt((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
            if (hours > 0) {
                return hours + "小时前";
            }

            var minutes = parseInt((distance % (1000 * 60 * 60)) / (1000 * 60));
            if (minutes > 0) {
                return minutes + "分钟前";
            };

            return "刚刚";
        }

        // 调用方法
        var time = new Date("2021-08-08 12:00:00").getTime();

        alert(getTimeDistance(time)) // => 2天前
    </script>


</head>
<body>

</body>
</html>