Here's a simple test I used in an attempt to recreate the problem, although was unable.
<%@ Page Language="C#" UICulture="en-GB" %>
<%@ Register assembly="Ext.Net" namespace="Ext.Net" tagprefix="ext" %>
<!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">
<head runat="server">
<title>Ext.NET Example</title>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:DateField ID="DateField1" runat="server" />
</form>
</body>
</html>
Which version of Ext.NET are you using 1.0, 1.1 or 1.2?
Can you modify the sample above to demonstrate the issue.