Some time ago I run into following problem. The date picker control in Dynamics AX 2009 SSRS reports used wrong date format (EN-US) while it should use NO (Norwegian). This concerned all AX 2009 SSRS reports, and viewing them from both Dynamics AX client and in Enterprise Portal. When viewing the report directly in reporting services (Report Manager) it works correctly. The version of SQL used was SQL 2005 SP3. What was more interesting all regional settings for each server (AOS, SQL, MOSS) where set up into Norwegian. We also checked registry keys for each user on Reporting Services server HKEY_USER\.Default\Control Panel\International and HKEY_USER \........\Control Panel\International had regional setting set up to Norwegian.

We never found caused of the problem however we managed to find workaround
We found out that when we run report from Report Server (using ReportViewer.aspx) the date picker is behaving in the same way. To solve it we set up Culture on Norwegian for ReportViewer.aspx in following way:

Edit the ReportViewer.aspx webpage. It usually in this directory: "%ProgramFiles%\Microsoft SQL Server\ MSSQL.\Reporting Services\ReportServer\Pages\"

Add Culture attribute to the @Page declaration like this:

<%@ Page Language="C#" AutoEventWireup="true" Inherits="Microsoft.ReportingServices.WebServer.ReportViewerPage" Culture="nb-NO"%>
It worked fine in this case, but if you have users connecting from Europe and US in the same time, US users will start complain that in their case they always get European format.