concatinating values in textbox

I have a problem with this code.

I am trying to perform validation on the three drop down menus which give the DATE, MONTH and the YEAR.

I am putting the values of each into a hidden text box then
performing validation on the text box.

However, i need to allow the user to change the drop down value (by pulling it down and
selecting a value) and change the textbox automatically.

It works if i select 1 date, 1 month then 1 year. But, if i change the year, month or date again it just adds
to the textbox.

for example,

works: 6/6/2005

then if i change the year again (without a refresh of the page) the box reads:


How can i stop this?

Help appreciated.

// Check the INFANT Birthday
// 2 Year Old Check

function checkBirthday(obj)
      if( !/^(\d{1,2})\/(\d{1,2})\/(\d{4})$/.test(obj.value) ) {
            alert( "Invalid date supplied - must be format DD/MM/YYYY" );
      var d = new Date();
      var d2 = new Date(RegExp.$3, RegExp.$2, RegExp.$1);
      var diff = d.getDiff(d2, "y")
      if( isNaN(diff) ) {
            alert( "Invalid date supplied" );
      else if( diff < 2 ) {
            alert( "This IS an INFANT \n they are younger that 2" );
            infant = true;
      else if( diff > 2 ) {
            alert( "This is NOT an INFANT \n they are older that 2" );
            infant = false;
            document.form.birthday.value = "";

<form name="form" action="abc.htm">
INF:<select name="date"
style="color: black; font: 7pt Verdana, Arial, Helvetica, sans-serif;">
      Response.write "<option> --- </option>"
For d = 01 to 31
      Response.write "<option value=" & d & "/>" & d & "</option>"
<select name="month"
style="color: black; font: 7pt Verdana, Arial, Helvetica, sans-serif;">
      Response.write "<option> --- </option>"
For m = 01 to 12
      response.Write "<option value=" & m & "/>" & MMtoMMM(m) & "</option>"
Dim three_years_ago
three_years_ago = year(date) - 3
<select name="year"
style="color: black; font: 7pt Verdana, Arial, Helvetica, sans-serif;">
      Response.write "<option> --- </option>"
For y = three_years_ago to year(date)
      Response.Write "<option value=" & y & ">" & y & "</option>"

<input type="text" name="birthday" size="5" style="font:7pt;" value="" maxLength="10">

<img src="images/required.gif"  onclick="javascript: checkBirthday(document.form.birthday);" value="Confirm">
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Change your onChange from:[].value;


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Be sure to change all three (date, month, and year).
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