The time zone is a string indicating the name of a time zone, one of:
* As used in the Olson time zone database (the "tz database" or
"tzdata"), such as "America/New_York"
* An absolute time zone offset of the form +XX:XX or -XX:XX, such as +07:30
Whether a timezone string is present indicates different semantics about
the data:
* If the time zone is null or equal to an empty string, the data is "time
zone naive" and shall be displayed *as is* to the user, not localized
to the locale of the user.