Nevertheless, you can use the get and set locale to change the information but if your program crashes while it is set to the wrong region, your customer won't like you. I'd advise doing it within some sort of try catch for each call so that no matter what happens, the region they have set will be reverted to whether the calculation from the date succeeds or fails.
To me, it's the height of arrogance for a developer to decide that the users must adhere to their particular vision of how data must be input. Remember that in these days of multi-tasking, your application isn't going to be the only one with which the user is interacting, and if you change personal settings, you're going to impact all active applications. (HINT: they're called "personal" settings for a reason!)
I would strongly recommend living with the users' chosen settings, rather than trying to dictate to them!
Doug Steele, Microsoft Access MVP
http://www.AccessMVP.com/djsteele (no e-mails, please!)
Co-author
Access Solutions — Tips, Tricks, and Secrets from Microsoft Access MVPs (ISBN 978-0-470-59168-0)