Time zones are user defined. (by default I believe?)

Look for the "Intelligent Search" to the top right and type in "Search Users" and click on this when it appears. When it goes to the Search Users screen you will see a list of all the users on your site. Click on your username or the username of anyone you want to set a timezone for and you will be taken to that users screen. Click "Edit User." From here there is an option to change the "Time Zone" of the user. Change this accordingly and click "Update User."

Hope this helps.