JQuery Code:

//Adjust date picker days in month.
$('select[name=birth_year],select[name=birth_month]').change(function(){
    
    var year = $('select[name=birth_year]').val();
    var month = $('select[name=birth_month]').val();

    var days_in_month = 31;

    if( month == 2 && year % 4 == 0 ){
        days_in_month = 29;
    }
    else if( month == 2 && year % 4 != 0 ){
        days_in_month = 28;
    }
    else if( $.inArray(parseInt(month,10), [4,6,9,11]) !== -1 ){
        days_in_month = 30;
    }

    var $days_selector = $('select[name=birth_day]');

    for( var i = $days_selector.children().length; i < days_in_month; i++ ){
        $days_selector.append('<option value="'+(i+1)+'">'+(i+1)+'</option>');
    }

    $days_selector.find('option:gt('+(days_in_month-1)+')').remove();
    
});

HTML Code:

<select name="birth_year">
		<option value="1970">1970</option>
		<option value="1971">1971</option>
		<option value="1972">1972</option>
		<option value="1973">1973</option>
		<option value="1974">1974</option>
		<option value="1975">1975</option>
		<option value="1976">1976</option>
		<option value="1977">1977</option>
		<option value="1978">1978</option>
		<option value="1979">1979</option>
		<option value="1980">1980</option>
		<option value="1981">1981</option>
		<option value="1982">1982</option>
		<option value="1983">1983</option>
		<option value="1984">1984</option>
		<option value="1985">1985</option>
		<option value="1986">1986</option>
		<option value="1987">1987</option>
		<option value="1988">1988</option>
		<option value="1989">1989</option>
		<option value="1990" selected>1990</option>
		<option value="1991">1991</option>
		<option value="1992">1992</option>
		<option value="1993">1993</option>
		<option value="1994">1994</option>
		<option value="1995">1995</option>
		<option value="1996">1996</option>
		<option value="1997">1997</option>
		<option value="1998">1998</option>
		<option value="1999">1999</option>
		<option value="2000">2000</option>
		<option value="2001">2001</option>
		<option value="2002">2002</option>
</select>
<select class="small" name="birth_month">
		<option value="1" selected>1</option>
		<option value="2">2</option>
		<option value="3">3</option>
		<option value="4">4</option>
		<option value="5">5</option>
		<option value="6">6</option>
		<option value="7">7</option>
		<option value="8">8</option>
		<option value="9">9</option>
		<option value="10">10</option>
		<option value="11">11</option>
		<option value="12">12</option>
</select>
<select class="small" name="birth_day">
		<option value="1" selected>1</option>
		<option value="2">2</option>
		<option value="3">3</option>
		<option value="4">4</option>
		<option value="5">5</option>
		<option value="6">6</option>
		<option value="7">7</option>
		<option value="8">8</option>
		<option value="9">9</option>
		<option value="10">10</option>
		<option value="11">11</option>
		<option value="12">12</option>
		<option value="13">13</option>
		<option value="14">14</option>
		<option value="15">15</option>
		<option value="16">16</option>
		<option value="17">17</option>
		<option value="18">18</option>
		<option value="19">19</option>
		<option value="20">20</option>
		<option value="21">21</option>
		<option value="22">22</option>
		<option value="23">23</option>
		<option value="24">24</option>
		<option value="25">25</option>
		<option value="26">26</option>
		<option value="27">27</option>
		<option value="28">28</option>
		<option value="29">29</option>
		<option value="30">30</option>
		<option value="31">31</option>
</select>