Lets see if I'm of any help here... Hmm... I'll also streamline the "year" list using Javascript.
Well, I'm a bit of a newb at this, so I don't know what the heck "return wait_for_load(this, event, function() { editor_date_month_change(this, 'birthday_day','birthday_year'); });" means. So, just put that back in if you need it for something.
Code:
<html>
<head>
<title>Test</title>
</head>
<body>
Date Of Birth:
<select name="month" onChange="changeDate(this.options[selectedIndex].value);">
<option value="na">Month</option>
<option value="1">January</option>
<option value="2">February</option>
<option value="3">March</option>
<option value="4">April</option>
<option value="5">May</option>
<option value="6">June</option>
<option value="7">July</option>
<option value="8">August</option>
<option value="9">September</option>
<option value="10">October</option>
<option value="11">November</option>
<option value="12">December</option>
</select>
<select name="day" id="day">
<option value="na">Day</option>
</select>
<select name="year" id="year">
<option value="na">Year</option>
</select>
<script language="JavaScript" type="text/javascript">
function changeDate(i){
var e = document.getElementById('day');
while(e.length>0)
e.remove(e.length-1);
var j=-1;
if(i=="na")
k=0;
else if(i==2)
k=28;
else if(i==4||i==6||i==9||i==11)
k=30;
else
k=31;
while(j++<k){
var s=document.createElement('option');
var e=document.getElementById('day');
if(j==0){
s.text="Day";
s.value="na";
try{
e.add(s,null);}
catch(ex){
e.add(s);}}
else{
s.text=j;
s.value=j;
try{
e.add(s,null);}
catch(ex){
e.add(s);}}}}
y = 2010;
while (y-->1909){
var s = document.createElement('option');
var e = document.getElementById('year');
s.text=y;
s.value=y;
try{
e.add(s,null);}
catch(ex){
e.add(s);}}
</script>
</body>
</html>
My only disclaimer is that I have no formal training in html or javascript, so if this isn't what you wanted, then sorry.
Edit: oops too late.
Edit again: Actually, his doesn't work properly.
Well, at least his code looks alot cooler than mine.
Add to del.icio.us 
Can you digg it?
Linear Mode
