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Did you make the customization for your USER setting (through edit profile) or for the SYSTEM setting (through Administration > System Admin > Preferences > General Settings). In the system setting you should be able to set the location of your server and a default user setting.
For example, my servers are mostly in New York (as I have determined through trial and error, even though my host is in California), and I am in Washington state. I have the server timezone set to GMT -5 and my default timezone to GMT -8. The default timezone is automatically the default for new users to your site unless they change it through their own user settings. BUT it's critical that the server timezone be set properly for any time to appear correctly on your site.
Does this help, or is this how you already have it set up?