Wybierz datę:

<form id="Formularz" method="post" action="skrypt_na_serwerze.php">
<fieldset>
<legend>Wybierz datę:</legend>
        <select name="Dzien" onchange="pokazValue()"></select>
        <select name="Miesiac" onchange="pokazValue()"></select>
        <select name="Rok" onchange="pokazValue()"></select>
</fieldset>
</form>

document.addEventListener("DOMContentLoaded", function() {
    var form = document.getElementById('Formularz');
    var month = ['styczeń','luty','marzec','kwiecień','maj','czerwiec','lipiec','sierpień','wrzesień','październik','listopad','grudzień'];
    var rok = (new Date()).getFullYear();
    var x;

    for (x=0; x<31; x++) {
        var day = (x < 9)? '0'+(x+1).toString() : x+1;
        var newOption = new Option(day, x+1, 0, 0);
        form.Dzien.options[x] = newOption;
    }

    for (x=0; x<12; x++) {
        var newOption = new Option(miesiace[x], x+1, 0, 0);
        form.Miesiac.options[x] = NowyOption;
    }

    for (x=0; x<3; x++) {
        var newOption = new Option(rok+x, rok+x, 0, 0);
        form.Rok.options[x] = newOption;
    }

    function showValue() {
        var day = form.Dzien.options[form.Dzien.selectedIndex].value;
        day = (day < 10)?'0'+day.toString() : day;

        var month = form.Miesiac.options[form.Miesiac.selectedIndex].value;
        month = (month < 10)?'0'+month.toString() : month;

        var year = form.Rok.options[form.Rok.selectedIndex].value;

        return day + '-' + month + '-' +year;
    }

    document.querySelector('select[name="Dzien"]').addEventListener('change', function() {
        alert(showValue());
    });
    document.querySelector('select[name="Miesiac"]').addEventListener('change', function() {
        alert(showValue());
    });
    document.querySelector('select[name="Rok"]').addEventListener('change', function() {
        alert(showValue());
    });
});