alert창 까지는 완성된 거 같은데..

날짜 계산하는 방법을 모르겠어요.. 달마다 다르고 년마다 다르고 ㅠㅠ

힌트 좀 주세요...

index.html

<!DOCTYPE html>
<html>
  <head>
    <title>Couple Story</title>
    <meta charset="utf-8" />
    <link href="css/styles.css" rel="stylesheet" />
  </head>

  <body>
    <div class="navbar">
      <img id="logo" src="images/logo.png" width="157">
    </div>

    <div class="main">
      <div class="container">
        <!-- 만난지 며칠? -->
        <h2>만난지 <span id="day-count">00</span>일째</h2>

        <div class="img-row">
          <img src="images/female.png" width="148">
          <img src="images/dot.png" width="8">
          <img src="images/dot.png" width="8">
          <img src="images/heart.png" width="16">
          <img src="images/dot.png" width="8">
          <img src="images/dot.png" width="8">
          <img src="images/male.png" width="148">
        </div>

        <!-- 만나기 시작한 날짜 -->
        <div class="date-row">
          <label for="start">시작일</label>
          <input type="date" name="start" id="start">
        </div>

        <!-- 며칠됐는지 계산하기! -->
        <button id="calculate">결과 보기</button>
      </div>
    </div>

    <script
      src="https://code.jquery.com/jquery-3.2.1.min.js"
      integrity="sha256-hwg4gsxgFZhOsEEamdOYGBf13FyQuiTwlAQgxVSNgt4="
      crossorigin="anonymous"></script>
    <script>
        $('#calculate').on('click', function(){
            function monthCalculate(month) {
                switch (month) {
                    case 0:
                        return 31;
                    case 1:
                        return 60;
                    case 2:
                        return 91;
                    case 3:
                        return 121;
                    case 4:
                        return 152;
                    case 5:
                        return 182;
                    case 6:
                        return 213;
                    case 7:
                        return 244;
                    case 8:
                        return 274;
                    case 9:
                        return 305;
                    case 10:
                        return 335;
                    case 11:
                        return 366;
                }
            }
            var startDate = new Date($('#start').val());
            var currentDate = new Date();
            var meetingDay;
            
            // 유효한 시작날짜인지 확인
            if (!$('#start').val()) {
                alert('시작 날짜를 입력해주세요!');
            }
            
            // 만난 날 계산하기
            console.log(startDate.getDate());
            var totalCurrentDate = currentDate.getDate() + monthCalculate(currentDate.getMonth()) + currentDate.getFullYear() * 365;
            var totalStartDate = startDate.getDate() + monthCalculate(startDate.getMonth()) + currentDate.getFullYear() * 365;
            meetingDay = totalCurrentDate - totalStartDate + 1;
            
            // 만난 날 수로 바꿔주기
            $('#day-count').text(meetingDay);
        });
        
        
    </script>
  </body>
</html>

css/styles.css

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-width: 992px;
  font-family: "Helvetica";
}

/* navigation bar */

.navbar {
  height: 66px;
  background-color: white;
  text-align: center;
  line-height: 66px;
}

#logo {
  vertical-align: middle;
}

/* main */

.main {
  background-image: linear-gradient(135deg, #B0A8FF 0%, #786FFF 100%);
  padding: 80px 0;
  min-height: calc(100vh - 66px);
}

.container {
  background: #FFFFFF;
  box-shadow: 2px 2px 4px 2px rgba(0,0,0,0.10);
  border-radius: 10px;
  width: 600px;
  margin: 0 auto;
  text-align: center;
  padding: 32px 66px;
}

.container h2 {
  font-size: 24px;
  color: #4A4A4A;
  margin: 0;
  margin-bottom: 28px;
}

.container h2 span {
  font-weight: 700;
  color: #786FFF;
}

.container .img-row {
  margin-bottom: 40px;
}

.container .img-row img {
  vertical-align: middle;
  margin: 0 6px;
}

.date-row {
  margin-bottom: 40px;
}

label[for=start] {
  font-size: 16px;
  color: #4A4A4A;
  line-height: 34.5px;
  vertical-align: middle;
  margin-right: 13px;
}

input[type=date] {
  border: 1px solid #786FFF;
  border-radius: 5px;
  padding: 8px 14px;
  vertical-align: middle;
  font-size: 16px;
}

#calculate {
  width: 100%;
  background: #786FFF;
  box-shadow: 0 2px 4px 0 rgba(0,0,0,0.50);
  border-radius: 5px;
  color: white;
  font-size: 18px;
  padding: 14px 0;
  border: none;
  cursor: pointer;
}