countdown.js

var endDate = new Date('2021/03/08 12:00:00');
var interval = 1000;


function countdownTimer(){
    var nowDate = new Date();
    var period = endDate - nowDate ;
    var addZero = function(n){return('0'+n).slice(-2);}
    var addZeroDay = function(n){return('0'+n).slice(-3);}
    
    if(period >= 0) {
        var day = Math.floor(period / (1000 * 60 * 60 * 24));
        period -=  (day　*(1000 * 60 * 60 * 24));
        var hour = Math.floor(period / (1000 * 60 * 60));
        period -= (hour *(1000 * 60 * 60));
        var minutes =  Math.floor(period / (1000 * 60));
        period -= (minutes * (1000 * 60));
        var second = Math.floor(period / 1000);
        var insert = "";
        insert += '<span class="h">' + day +'<span class="text-3xl">日</span>' + '</span><br class="md:hidden">';
        insert += '<span class="h">' + hour + '<span class="text-3xl">時間'+'</span>';
        insert +=  '<span class="m">' + minutes +'<span class="text-3xl">分' + '</span>';
        insert += '<span class="s">' + addZero(second)+ '<span class="text-3xl">秒'+ '</span>';
        document.getElementById('result').innerHTML = insert;
        setTimeout(countdownTimer,10);
    }

    else{
        var insert = "";
        var number = 0;
        insert += '<span class="h">' + number + number + '</span>';
        insert +=  '<span class="m">' + number + number + '</span>';
        insert += '<span class="s">' + number + number + '</span>';
        document.getElementById('result').innerHTML = insert;
    }
}

countdownTimer();