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The most recent zoneinfo database, as maintained by the Internet Assigned Numbers Authority, holds two new prefixes, 'posix' and 'right'. For example where there used to be just Asia/Kolkata the new database has added posix/Asia/Kolkata and right/Asia/Kolkata.

This database is also known as the Olson database after its first developer, or the tz database.

What do these newly added prefixes mean, and what's their practical effect? Can any of them safely be filtered out of timezone-choice picklists presented to users?

Globalized web apps (such as WordPress) use these zoneinfo names for user-preference picklists. They're in MySQL's timezone support setup.

O. Jones
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2 Answers2

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right (or also leap, also zoneinfo-leaps) is about using times including leap seconds. Posix (also zoneinfo-posix, often just `zoneinfo) is about using POSIX times, so without considering leap seconds.

In theory you should not choose, the system choose it for you, considering how the time is stored in the system.

But no, for user you should not use data in zoneinfo.

To quote Python documentation:

Note

These values are not designed to be exposed to end-users; for user facing elements, applications should use something like CLDR (the Unicode Common Locale Data Repository) to get more user-friendly strings. See also the cautionary note on ZoneInfo.key.

Note: some zones are obsolete, and the text is simple ASCII, so it cannot represent correctly the city names, and the Englished version is not always the best one.

Giacomo Catenazzi
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The right/ prefix marks timezones taking leap seconds into account.

The posix/ prefix marks timezones using, well, POSIX time. Those timezones are, practically, the same as the unprefixed ones.

It's probably fine to filter out the prefixed names when presenting timezone choices to users (the way WordPress, for example, does). If you're an astronomer your parsecage may vary.

O. Jones
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