index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Date calculator</title>
    <link rel="stylesheet" href="calender.css">
</head>
<body>
    <script src="calender.js"></script>
    
    <div class="main">
        <form>
            <div>
                <label>Start Date</label><br>
                
                <input type="date" onchange="showDate()" id="startDate">
                <br>
                <div id="firstinput">

                </div>

                <div><br></div>

                <label>Operation</label><br>
                
                <select id="operation">
                    <option value="add">Add</option>
                    <option value="subtract">Subtract</option>
                </select>

                <div class="wrap-up">
                    <div>
                       
                        <input type="number" id="durationValue" min="0" placeholder=" Please enter value for operation"><br>
                    </div>
                    <div>
                        <select id="durationType">
                            <option value="days">Days</option>
                            <option value="months">Months</option>
                            <option value="weeks">Weeks</option>
                            <option value="years">Years</option>
                        </select>
                    </div>
                </div>
            </div>
            <br>

            <button type="button" onclick="output()">Submit</button>
        </form>

        <div id="result"></div>

        <div id="today" ><div>
    </div>
</body>

</html>