1. Open your Google Profile page.
2. Click the edit button next to Personal Information
(edit your personal info without creating a public profile.)
3. You will get a page where you can edit your Gmail name address and Time zone etc.
4. Click on Display all timezones
5. Click the Time zone drop down and choose the Time zone you want in your Gmail messages
6. Save the changes
You can also add or add other information if you want to change.