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                <p>来年まで<span id="timer"></span>秒</p>


  font-size: 20px;
  font-weight: bold;
  color: red;


                'use strict';

// nextYearと今の間の秒数を取得して#timerに表示
function getSecond(nextYear){
  let now = new Date();
  document.getElementById('timer').textContent = Math.floor((nextYear.getTime() - now.getTime()) / 1000);
// 今の時間を作成
let now = new Date();
// 来年の元旦0:00を作成
//第2引数は1月だけど0 - 11で指定するので1引いてる
let nextYear = new Date(now.getFullYear() + 1, 0, 1);

// 1秒毎に来年元旦までの秒数を表示
// コールバック関数に引数を渡すため
// 無名関数を利用している
setInterval(function(){getSecond(nextYear)}, 1000);