Any suggestions what OS works best on a file server?

Changing the time of the server isn't possible in this case.

I've also tried to add SetEnv TZ EET and SetEnv TZ Europe/Helsinki to .htaccess with no result.
I also understand I could use php function date_default_timezone, but don't know what file(s) I should add it on.

How can I change the time used by the site? I need to have upload/download, registering, payment, expiring-time etc. displayed according to my target area (GMT +2 hours ). Any ideas how I could do this? In future versions it would be good to have an option for the users to choose the time zone thems...