[tz] Beginner's help request

Daniel Ford dfnojunk at gmail.com
Wed Oct 18 12:58:01 UTC 2017

Paul & Tim, thank you for your examples and explanations.  I'm finally
starting to get an understanding of the scope of the complex problem.  I've
also read some of the 'explanatory' files to better understand the database
contents, POSIX format, etc, and I think I'm closer to understanding how to
use the database for my clock.

It's really interesting that some countries/localities still make short-term
changes to DST start/stop dates, and that some areas experience four changes
in a year!  My first reaction was: "I won't offer my clock to those
countries", but then I thought: "But they're the ones who need it even more
than me!"

So I need to try to deal with those weird rule changes in my clock.  My
first thought is: "How does the TZ database record rules when they change
four times in a year rather than two?"  My next problem was "where do I find
these countries in the database, since there's no 'MiddleEast' file?"  But
then I was alerted to Zone1970.tab, and found that they were included in the
'Asia' file.

So I went looking for a place where these quadruple rules might apply.  My
first search was for Baghdad, which turned up another issue in the current
(2017b) Asia file that doesn't quite make sense to me.  From line 1146...

Rule	Iraq	1986	1990	-	Mar	lastSun	1:00s	1:00	D
# IATA SSIM (1991/1996) says Apr 1 12:01am UTC; guess the ':01' is a typo.
# Shanks & Pottenger say Iraq did not observe DST 1992/1997; ignore this.
Rule	Iraq	1991	2007	-	Apr	 1	3:00s	1:00	D
Rule	Iraq	1991	2007	-	Oct	 1	3:00s	0	S
Zone	Asia/Baghdad	2:57:40	-	LMT	1890
			2:57:36	-	BMT	1918     # Baghdad Mean
			3:00	-	+03	1982 May
			3:00	Iraq	+03/+04

Because there is no *current* RULE for Baghdad (the last expired 2007
according to the table), I assume that DST no longer occurs there.  But Bill
Seymour's explanation of the digits in the FORMAT column are 'standard time
name/daylight saving time name'.  Given that DST no longer applies (if my
Rule interpretation is correct) and the standard GMT offset is +3h, why
would these (now identical?) zones be named +03 and +04?

At this stage I haven't continued on my search for a quad-change locality
(I'll wait till I better understand the 'simple' ones, such as the above,
first!), but if anyone can direct me to a quad-change rule, it would save me
some time further down the track.


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