Select boxes for birthday in freemember echos out bday_d , bday_m, bday_y for expression engine. Works with PHP on output.

freemember-bday-selectors.php

<div class="control-group">
    <label for="{pr_form_id}-date-of-birth">Date of Birth</label>

    <select id="{embed:form_prefix}-bday_d" name="bday_d" class="dob-day">
        <option value="">Day</option>
        <?php for ($i=1; $i < 32; $i++) : ?>
        <option value="<?php echo $i ?>"
            <?php echo ($i == "{bday_d}") ? 'selected="selected"' : "" ?>
            >
            <?php echo $i ?></option>
        <?php endfor; ?>
    </select>
    <select id="{embed:form_prefix}-bday_m" name="bday_m" class="dob-month">
        <option value="">Month</option>
        <?php foreach(array('January','February','March','April','May','June','July','August','September','October','November','December') as $k => $i) : ?>
        <option
            value="<?php echo ($k+1) ?>"
            <?php echo (($k+1) == "{bday_m}") ? 'selected="selected"' : "" ?>
            >
            <?php echo $i ?>
        </option>
        <?php endforeach; ?>
    </select>
    <select id="{embed:form_prefix}-bday_y" name="bday_y" class="dob-year">
        <option value="">Year</option>
        <?php
            $max_age = 100;
            $min_age = 1;
            $current = date('Y');
        ?>
        <?php foreach (range($min_age,$max_age) as $year) : ?>
                <option
                    value="<?php echo ($current - $year) ?>"
                    <?php echo (($current - $year) == "{bday_y}") ? ' selected="selected" ' : "" ?>
                >
                    <?php echo ($current - $year) ?>
                </option>
            <?php endforeach; ?>
        ?>
    </select>
</div>

Will you include filter for leap year?