usertimezone( user )
Returns the site primary timezone if the application is configured to override user preferences; otherwise it returns the preferred timezone of the given user or the site primary timezone if the user doesn't have a preference set.
The user (or the username of the user) for whom the timezone should be returned.
The timezone is returned as a string, such as "GMT" or "EST".
You can copy and paste these examples into the Expression Rule Designer to see how this works.