[tz] Vanguard vs. "main" vs. rearguard
Kerry Shetline
kerry at shetline.com
Wed May 19 19:33:03 UTC 2021
As far as I can tell, their are only four sections in the current tzdb marked as vanguard/rearguard, two sections in the europe for Europe/Dublin and Europe/Prague, and two in the africa file for Africa/Windhoek.
What I’m confused about is references I’ve seen to a “main” format, like this:
> Changes to future timestamps
>
> Guess that Morocco will continue to fall back just before and
> spring forward just after Ramadan, the practice since 2012.
> (Thanks to Maamar Abdelkader.) This means Morocco will observe
> negative DST during Ramadan in main and vanguard formats, and in
> rearguard format it stays in the +00 timezone and observes
> ordinary DST in all months other than Ramadan…
At least from the current examples, it looks like one would have to choose either Vanguard or Rearguard, period. There is no middle ground of “main” when I look at a case like this below:
# Zone NAME STDOFF RULES FORMAT [UNTIL]
Zone Europe/Dublin -0:25:00 - LMT 1880 Aug 2
-0:25:21 - DMT 1916 May 21 2:00s
-0:25:21 1:00 IST 1916 Oct 1 2:00s
0:00 GB-Eire %s 1921 Dec 6 # independence
0:00 GB-Eire GMT/IST 1940 Feb 25 2:00s
0:00 1:00 IST 1946 Oct 6 2:00s
0:00 - GMT 1947 Mar 16 2:00s
0:00 1:00 IST 1947 Nov 2 2:00s
0:00 - GMT 1948 Apr 18 2:00s
0:00 GB-Eire GMT/IST 1968 Oct 27
# Vanguard section, for zic and other parsers that support negative DST.
1:00 Eire IST/GMT
# Rearguard section, for parsers lacking negative DST; see ziguard.awk.
# 1:00 - IST 1971 Oct 31 2:00u
# 0:00 GB-Eire GMT/IST 1996
# 0:00 EU GMT/IST
# End of rearguard section.
If there is a middle ground here, neither Vanguard or Rearguard, what is it? And if there are no current examples with such a middle ground, what would a middle ground of neither Vanguard or Rearguard look like?
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