Time library discussion (reprise)

Carl Witty cwitty at newtonlabs.com
Thu Nov 13 12:31:52 EST 2003

On Thu, 2003-11-13 at 02:16, Simon Marlow wrote:
> I think you're assuming that the timezone TLAs don't represent fixed
> offsets relative to UTC.  I assume that they do.
> For example, here we are in timezone GMT at the moment (== UTC+0000),
> but we switch to BST in the summer (UTC+0100).
> I just looked up the list of timezone abbreviations, and it's pretty
> short.  The library could easily include this list.

Where did you find this list?  With a little Google searching I found
several conflicting sources:

has MANY meanings for some TLA's: (AST could be Al Manamah, Anguilla,
Antigua, Antilles, Arabia, Arabic, or Atlantic Standard Time; or Asia
Time).  Brazil is listed has having BRT for Brasilia Time (for the
middle and east) or for Brasil Time (for the south) or BRST for the
south "Summer Time".

lists for Brazil "Atlantic time", "Central Brazil time", "Amazone time",
and "Acre time", with no mentioned abbreviations.

says that BST is British Summer, and that BZT2 is Brazil Zone 2 (no
mention of other Brazil zones); but mentions at the bottom that BST may
also be Brazil Standard.

lists information retrieved from SysVR4 timezone files; here, EST could
mean Canada/Eastern or US/Eastern (-5:00), Brazil/East (-3:00),
Chile/EasterIsland (-6:00), or Australia/Tasmania (+10:00).

The data which is currently "authoritative", at least for Linux (the
ultimate source for the files in my /usr/share/zoneinfo directory), is
the tzdata package from ftp://elsie.nci.nih.gov/pub/.  The latest
version (as of this writing), tzdata2003d.tar.gz, is less ambiguous than
the SysVR4 data, but still contains ambiguities (a quick check reveals
EST for Australia (+10:00) and the US (-5:00); there's actually some
discussion of the issue, in the file "australasia", which mentions IST:
Israel Standard Time (+2:00), India Standard Time (+5:30), Irish Summer

> Perhaps we want a way to map geographical locations to timezones.

Ooh, dangerous.  Microsoft took its cool timezone world map out of
post-Windows 95 (I think) editions because of political issues: some
territory was disputed between two countries, who wanted to apply
different timezones to the territory; so Microsoft's map was seen as
endorsing one country as the "correct" owner of the territory.  (I don't
remember the details.)

Would you have, like tzdata does, a separate "time zone" for Boise,
Idaho?  Boise is in Mountain time; but, according to tzdata, it switched
into or out of daylight savings time four weeks late in 1974 (I was
there at the time, but I was only 5, so I don't remember), so it's
listed separately (in case somebody wants to know what the local time
would have been in 1974, I guess).

I recommend reading through the files in tzdata for a LOT of interesting
time zone anecdotes (things like "Shanks partitions Indiana into 345
regions, each with its own time history, and writes ``Even newspaper
reports present contradictory information.''" and "I know one story of a
town on one time zone having its school in another, such that a mom had
to serve her family lunch in two shifts."

Carl Witty

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