[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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