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<!DOCTYPE html>
<!--
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<html>
    <head>
        <title>TODO supply a title</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>
    <body>
        <select id="miesiac">
            <option value="0">Styczeń</option>
            <option value="1">Luty</option>
            <option value="2">Marzec</option>
            <option value="3">Kwiecień</option>
            <option value="4">Maj</option>
            <option value="5">Czerwiec</option>
            <option value="6">Lipiec</option>
            <option value="7">Sierpień</option>
            <option value="8">Wrzesień</option>
            <option value="9">Październik</option>
            <option value="10">Listopad</option>
            <option value="11">Grudzień</option>
        </select>
        <input id="year" min="2018" max="2019" type="number">
        <input type="button" value="Pokaż" onclick="calendar()">
        <p id="cal"></p>
        <script>
            var tab = ['pon', 'wt', 'śr', 'czw', 'pt', 'sob', 'ndz'];
            function calendar()
            {
                var month = document.getElementById('miesiac').value;
                var year = document.getElementById('year').value;
                t = new Date(year, month, 1);

                var m = t.getMonth();
                if(m+1 == 12) m = -1;
                console.log(m);

                t.getDay();
                t = new Date(t.getTime()-(t.getDay()-1)*24*60*60*1000);
                //document.write(t);

                var html = "<table>";
                html = html + "<tr>";
                for(i=0; i<tab.length; i++)
                {
                    html = html + "<td>" + tab[i] + "</td>";
                }
                html = html + "</tr>";

                while(t.getMonth() != m+1)
                {
                    console.log(t.getMonth())
                    html = html + "<tr>";
                    for(let i=0; i<7; i++)
                    {
                        html = html + "<td>" + t.getDate() + "</td>";
                        t = new Date(t.getTime()+24*60*60*1000);
                    }
                    html = html + "</tr>";
                }
                html = html + "</html>";
                var p = document.getElementById('cal');
                cal.innerHTML = html;
                //document.write(html);
            }
        </script>
    </body>
</html>