JavaScript の日付処理でさっとやったやつをメモしておく
フォーマット、日付計算とかもっと良いのないかなと感じてます．．．
変数名も適当すぎる．．．

// 現在日時取得
const now = new Date();
// 今週末
const nextWeekEnd = new Date(now.getFullYear(), now.getMonth(), now.getDate() + (6 - now.getDay()));
// 来年初日
const nextYearFirst = new Date(now.getFullYear() + 1, 0, 1);

console.log(dateFormat(now));
console.log(diffDate(now, nextWeekEnd));
console.log(diffDate(now, nextYearFirst));
console.log(diffTimes(now, nextWeekEnd));
console.log(diffTimes(now, nextYearFirst));

// 曜日の日本語リスト
//const weekdays = ['日', '月', '火', '水', '木', '金', '土'];

// 日付フォーマット YYYY/MM/DD(a)HH:MI:SS
function dateFormat(date) {
    const formatOptions = {
        year: "numeric", month: "2-digit", day: "2-digit", weekday: 'short'
        , hour: "2-digit", minute: "2-digit", second: "2-digit"
    }
    return date.toLocaleString("ja-JP", formatOptions);
    // return date.toLocaleString("sv-SE").replace(' ', `(${weekend[date.getDay()]})`);
    // return date.getFullYear() + '/'
    //     + String(date.getMonth() + 1).padStart(2, '0') + '/'
    //     + String(date.getDate()).padStart(2, '0') + '('
    //     + weekend[date.getDay()] + ')'
    //     + String(date.getHours()).padStart(2, '0') + ':'
    //     + String(date.getMinutes()).padStart(2, '0');
}

// 日時の日数差
function diffDate(from, to) {
    const f = new Date(from.getFullYear(), from.getMonth(), from.getDate());
    const t = new Date(to.getFullYear(), to.getMonth(), to.getDate());
    return Math.floor((t.getTime() - f.getTime()) / (1000 * 60 * 60 * 24));
}

// 日時の時分秒差
function diffTimes(from, to) {
    const diffTime = to.getTime() - from.getTime();
    return {
        time: diffTime,
        hour: Math.floor(diffTime / (1000 * 60 * 60)),
        minute: Math.floor(diffTime / (1000 * 60)),
        Second: Math.floor(diffTime / 1000)
    }
}