Проверяйте код. Вот тестовый пример:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>Без имени</title>
<meta http-equiv="content-type" content="text/html;charset=utf-8" />
<meta name="generator" content="Geany 0.17" />
</head>
<style type="text/css"></style>
<script type="text/javascript">
<!--
function init() {
var today = new Date();
var day= today.getDate();
var days = 31;
var myday = document.getElementById('_select');
for ( var i = 1; i < days+1; i++) {
var newoption = document.createElement('option');
newoption.text = i;
newoption.value=i;
myday.add(newoption,null);
if ( i == day) {newoption.selected=true;}
}
}
//-->
</script>
<body onload="init()">
<select id="_select"></select>
</body>
</html>
UPD
Попробуйте добавлять так:
try {
currentSelect.add(newOption, null); // standards compliant
} catch(ex) {
currentSelect.add(newOption); // IE only
}