index.html

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    <title>Sex calculator</title>

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    <script src="./date-input.js"></script>

    <style>
        h1, p {
            font-family: "Open Sans", sans-serif;
            font-weight: 400;
            font-style: normal;
        }

        h1 {
            font-size: 2em;
        }

        p {
            font-size: 1.1em;
        }

        section {
            padding-left: 1em;
            display: flex;
            flex-direction: column;
        }

        .date-input {
            display: inline;
        }

        input, button {
            padding: 10px;
            font-size: 2em;
        }

        button {
            cursor: pointer;
        }
    </style>
</head>
<body>
<main>
    <section>
        <h1>When did your parents have sex?</h1>
        <p>This is the dumbest idea I've ever built. But hey, now you know when your parens had sex in order to conceive you!</p>
    </section>

    <section>
        <form>
            <p>Step 1: enter your birthday</p>
            <app-date-input id="date_input"></app-date-input>
            <p>Step 2:</p>
            <button type="submit">Calculate!</button>
            <p>Step 3: <span id="result">Get results!</span></p>
        </form>
    </section>
</main>

<script>
    document.forms[0].addEventListener('submit', (event) => {
        event.preventDefault();

        goGoGadgetCalculate();

        return false;
    });

    function goGoGadgetCalculate() {
        const input = document.getElementById('date_input');
        const sexDate = input.calculateSexDate();
        const sexMonth = sexDate.toLocaleString('default', {month: 'long'});

        window.result.innerHTML = `Your parents most likely fucked in <em>${sexMonth}</em>`;
    }
</script>
</body>
</html>