form.html

<form action="" method="post">
    <fieldset>
        <legend>Personal details</legend>
        <!-- name etc here -->
    </fieldset>
    <fieldset>
        <legend>Date of birth</legend>
        <label for="day">Day</label>
        <select name="day" id="day"><!-- options here --></select>
        <label for="month">Month</label>
        <select name="month" id="month"><!-- options here --></select>
        <label for="year">Year</label>
        <select name="year" id="year"><!-- options here --></select>
    </fieldset>
    <div><input type="submit"></div>
</form>