HTML list/menu connected to a field

Hi guys,

Yesterday I wrote a thread about an issue I have doing a contact form with specific rules. Someone gave me a good idea but I don’t know how to do it. The solution for my previous problem is to connect the 3 list/menu (month/day/year) to an hidden field, this way I can send only 1 data with 1 name. I need to do the work in flash and html, but I already know how to do it in flash (AS3) but not in html.

In AS3 I could simply tell;

txtContactBDay.text = date1.value + date2.value + date3.value

But how can I script this in HTML ???

<input type=“hidden” name=“txtContactBDay” value="???">

<select name=“date1”>
<option value=“na”>Month</option>
<option value=“1/12/1985”>January</option>…

<select name=“date2”>
<option value=“na”>Day</option>
<option value=“1/”>1</option>…

<select name=“date3”>
<option value=“na”>Year</option>
<option value=“2009”>2009</option>…

Thanks in advance