SOURCE

function countDown(time) {
    let startTime = new Date()
    let endTime = new Date(time)

    let leftTime = endTime.getTime() - startTime.getTime()
    let days = Math.floor(leftTime / (1000 * 60 * 60 * 24)),
        hours = Math.floor(leftTime / (1000 * 60 * 60) % 24),
        mins = Math.floor(leftTime / (1000 * 60) % 60),
        secs = Math.floor(leftTime / 1000 % 60);

    return `${days}天${hours}小时${mins}分${secs}秒`
}

let end = '2021/11/11'
let time = document.getElementsByClassName('main')[0]
let tip = document.getElementsByClassName('tip')[0]

tip.innerHTML = `距离${end}还有：`
setInterval(() => {
    time.innerHTML=countDown(end)
}, 1000)

<div class="time">
    <div class="tip"></div>
    <div class="main"></div>
</div>

.time {
    width: 50%
}

.time .main {
    position: relative;
    width: 100%;
    height: 200px;
    line-height: 200px;
    text-align: center;
    font-size: 30px;
    border: 1px solid gray;
}

.time .tip {
    position: absolute;
    width: 300px;
    height: 20px;
    margin: 0 auto;
    top: 10px;
    font-size: 14px
}