Preferences has an option to fill in your time zone offset from the browser, but this is a one-time thing. I think we should instead detect whenever it's different from preferences, and give them an option to automatically set the preference then (or hide the message). They wouldn't have to actually go to Special:Preferences, just click 'Yes' or something.
This allows respecting the preference if someone's OS/browser time zone is not what they prefer to use for some reason.
This is the solution Google Calendar uses.