d-day 오류

<!DOCTYPE html>
<html lang="ko">
<head>
  <script src="./js/calculator.js"></script>
  <link rel="stylesheet" href="./calculator.css">

  <title>D-Day</title>
</head>
<body>
  <h1>D-day</h1>
  <div id="d__day__container">
    <div class="d__day__child__container">
    <span id="days">0</span>
    <span>일</span>
    </div>
    <div class ="d__day__child__container">
      <span id="hours">0</span>
      <span>시간</span>
    </div>
    <div class ="d__day__child__container">
      <span id="min">0</span>
      <span>분</span>
    </div>
    <div class ="d__day__child__container">
      <span id="sec">0</span>
      <span>초</span>
    </div>
  </div>
  <div id="target-selector">
    <input id="year" type="text" placeholder="년도" class="day-caleander" size=5 maxlength="4">
    <input id="month" type="text" placeholder="월" class="day-caleander" size=5 maxlength="2">
    <input id="day" type="text" placeholder="일" class="day-caleander" size=5 maxlength="2">
  </div>
  <div id="target__selector">
    <button onclick="counterMake()" id="start__btn">카운트다운</button>
  </div>
</body>
</html>


const dateFormMaker = () => {
  const inputYear = document.getElementById("year").value;
  const inputMonth = document.getElementById("month").value;
  const inputDay = document.getElementById("day").value;

  const dateFormat = `${inputYear} - ${inputMonth} - ${inputDay}`;
  
  return dateFormat;
}

const counterMake = () => {
  const targetDateInput = dateFormMaker();
  const nowDate = new Date()
  const targetDate = new Date(targetDateInput);
  const remaining = (targetDate-nowDate) / 1000;
  if(remaining <=0) {
    console.log("타이머가 종료되었습니다.")
  }else if (isNaN(remaining)) {
    "유효한 시간 형식이 아닙니다"

  }
  
  const reaminingDate = Math.floor(remaining /3600 /24);
  const remainingHours = Math.floor (remaining/3600) & 24;
  const remainingMin = Math.floor(remaining/60) % 60;
  const remainingSec = Math.floor(remaining) & 60;

  const dDayCounter = `남은 시간 : ${reaminingDate}일 ${remainingHours}시 ${remainingMin}분 ${remainingSec}초`;
  console.log(dDayCounter)
  return dDayCounter;
  
}

console 하나하나 찍어가며 파악한 결과 targetDate에서 NaN이 뜨는데, 아무리 봐도 이유를 모르겠습니다 ㅠㅠㅠ

input 박스에 적은 값들도 잘 읽어오는데 갑자기 저기서 에러가 뜨니까 답답하네요 .

피드백 부탁드립니다.

인프런 커뮤니티 질문&답변