You are correct, timezone is not part of the EXIF specification although exiftool will let you write it. The only way to write timezone information in a way that is 100% compatible with the EXIF specification is to use the GPSTimeStamp tag, which is specified in UTC. Therefore the timezone can be deduced as the difference between this and the other times in the image. Although this is allowed by the specification, I don't think there is any software out there that will take advantage of this to determine the time zone.

- Phil