【GAS、JavaScript】月末や月またぎも考慮した日付の計算をする

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const today = new Date();
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let year = today.getFullYear();
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let month = today.getMonth(); // 注意: 月は0から始まる
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let day = today.getDate();
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console.log(today)
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console.log(year)
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console.log(month)
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console.log(day)
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today.setFullYear(2025);
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today.setMonth(11); // 12月
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today.setDate(25);
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console.log(today)
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console.log(today.getMonth())
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console.log(today.getDate())

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/**
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 * @description Get boolean value whether the difference between formerDay and latterDay is over or not.
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 * @param {Date()} formerDay
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 * @param {Date()} latterDay
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 * @param {number} targetDiff
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 * @return {boolean}
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*/
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function isDateDiffOver(formerDay, latterDay, targetDiff){
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  if(Math.abs(latterDay.getMonth() - formerDay.getMonth()) > 2){
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    console.log(`isDateDiffOver: 555555555555555555555555555555555555555555555555555555555555555555555`);
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    return true;
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  }else if(Math.abs(latterDay.getMonth() - formerDay.getMonth()) === 1){
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    let diffDates = latterDay.getDate() - formerDay.getDate();
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    formerDay.setDate(0); // Get end of month
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    diffDates += formerDay.getDate();
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    console.log(diffDates);
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    console.log(`isDateDiffOver: 666666666666666666666666666666666666666666666666666666666666666666666`);
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    if(diffDates <= targetDiff){
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      return true;
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    }
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  }else{
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    if(latterDay.getDate() - formerDay.getDate() <= targetDiff){
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      console.log(`isDateDiffOver: 77777777777777777777777777777777777777777777777777777777777777777777777`);
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      return true;
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    }
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  }
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  return false;
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}

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/**
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 * @description Get boolean value whether the difference between formerDay and latterDay is over or not.
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 * @param {Date()} formerDay
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 * @param {Date()} latterDay
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 * @param {number} targetDiff
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 * @return {boolean}
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 * @example
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 * formerDay = 2022/01/23; latterDay = 2022/01/24; targetDiff = 1; return false;
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 * formerDay = 2022/01/23; latterDay = 2022/01/24; targetDiff = 0; return true;
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*/
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function isDateDiffOver(formerDay, latterDay, targetDiff){
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  let timestamp1 = Math.floor(formerDay.getTime() / 1000);
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  let timestamp2 = Math.floor(latterDay.getTime() / 1000);
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  const margin = 10 * 1000;
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  console.log(formerDay)
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  console.log(latterDay)
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  console.log(targetDiff)
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  console.log(`isDateDiffOver: 11111111111111111111111111111111111111111111111111111111111111111`);
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  console.log(timestamp2 - timestamp1)
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  console.log(60 * 60 * 24 * (targetDiff + 1) - 1)
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  if(Math.abs(timestamp2 - timestamp1) >= 60 * 60 * 24 * targetDiff - margin){
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    console.log(`isDateDiffOver: 44444444444444444444444444444444444444444444444444444444444444444`);
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    return true;
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  }
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  console.log(`isDateDiffOver: 999999999999999999999999999999999999999999999999999999999999999999999`);
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  return false;
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}

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/**
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 * @description Get boolean value whether latterDay is later than formerDay or not.
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 * @param {Date()} formerDay
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 * @param {Date()} latterDay
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 * @return {boolean}
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 * @example
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 * formerDay = 2022/01/25; latterDay = 2022/01/24; return false;
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 * formerDay = 2022/01/23; latterDay = 2022/01/24; return true;
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*/
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function isLatterDayLater(formerDay, latterDay){
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  let timestamp1 = Math.floor(formerDay.getTime() / 1000);
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  let timestamp2 = Math.floor(latterDay.getTime() / 1000);
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  if(timestamp2 - timestamp1 > 0){
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    console.log(`isLatterDayLater: 88888888888888888888888888888888888888888888888888888888888888888888888888`);
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    return true
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  }
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  console.log(`isLatterDayLater: 999999999999999999999999999999999999999999999999999999999999999999999`);
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  return false;
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}