效果圖

程式碼片 直接copy就可以呼叫

<div id="timer"></div>

setInterval("leftTimer(2020,1,1,00,00,00)", 1000);
    //倒計時
function leftTimer(year, month, day, hour, minute, second) {
    var leftTime = (new Date(year, month - 1, day, hour, minute, second)) - (new Date()); //計算剩餘的毫秒數
    var days = parseInt
 
(leftTime / 1000 / 60 / 60 / 24, 10); //計算剩餘的天數
    var hours = parseInt(leftTime / 1000 / 60 / 60 % 24, 10); //計算剩餘的小時
    var minutes = parseInt(leftTime / 1000 / 60 % 60, 10); //計算剩餘的分鐘
    var seconds = parseInt(leftTime / 1000 % 60, 10); //計算剩餘的秒數
    days = checkTime(days);
    hours = checkTime(hours);
    minutes = checkTime(minutes)
 
;
    seconds = checkTime(seconds);
    document.getElementById("timer").innerHTML = days + "天" + hours + "小時" + minutes + "分" + seconds + "秒";
}

function checkTime(i) { //將0-9的數字前面加上0，例1變為01
    if(i < 10) {
        i = "0" + i;
    }
    return i;
}