Newbie questions... regarding tzdata199e.tar.gz

[Jon R. Kibler]

(If this is now where I should be asking these questions, I apologize -- please direct me to the correct source.)


I am new at time zone information, so please bear with me. 

I have to develop a "Y2K Rollover Model." Looking for time zone information, I found the tzdata199e.tar.gz archive (at ftp://elsie.nci.nih.gov/pub/) -- which is great -- exactly what I needed. I believe that I understand most of it, but I have a few questions. Most of them are about the fine points of how the rules work:


1) From 'africa', Sierra Leone has the following entry:
> # Sierra Leone
> # Rule NAME	FROM	TO	TYPE	IN	ON	AT	SAVE	LETTER/S
> # Whitman gives Mar 31 - Aug 31 for 1931 on; go with Shanks.
> Rule	SL	1935	1942	-	Jun	 1	0:00	0:40	SLST
> Rule	SL	1935	1942	-	Oct	 1	0:00	0	WAT
> Rule	SL	1957	1962	-	Jun	 1	0:00	1:00	SLST
> Rule	SL	1957	1962	-	Sep	 1	0:00	0	GMT
> # Zone NAME		GMTOFF	RULES	FORMAT	[UNTIL]
> Zone	Africa/Freetown	-0:53:00 -	LMT	1882
> 			-0:53:00 -	FMT	1913 Jun # Freetown Mean Time
> 			-1:00	SL	%s	1957
> 			 0:00	SL	%s

1a) It is my interpretation that no rules apply after Sep 1, 1962 -- is this a correct interpretation? If not, please explain.


1b) If no rules apply, then what letters are used for SL time? (As I interpret it, %s would be blank if no rules apply. If this is not the case, please explain.)


1c) If no rules apply, why not specify a "no rules" entry, such as the following? (I would think it would be much easier [less ambiguous] for HUMANS to read, and thus less likely to contain mistakes.)
> # Zone NAME		GMTOFF	RULES	FORMAT	[UNTIL]
> Zone	Africa/Freetown	-0:53:00 -	LMT	1882
> 			-0:53:00 -	FMT	1913 Jun # Freetown Mean Time
> 			-1:00	SL	%s	1957
> 			 0:00	SL	%s	1962 Sep 1
> 			 0:00	-	GMT


2) Please explain a "Numeric RULE", such as in the following example for Cape Verde:
> # Zone NAME		GMTOFF	RULES	FORMAT	[UNTIL]
> Zone Atlantic/Cape_Verde -1:34:04 -	LMT	1907			# Praia
> 			-2:00	-	CVT	1942 Sep
> 			-2:00	1:00	CVST	1945 Oct 15	# <<< Please explain
> 			-2:00	-	CVT	1975 Nov 25 2:00
> 			-1:00	-	CVT



3) Please explain "TYPE", such as in the following rule for South Australia:
> # Rule NAME	FROM	TO	TYPE	IN	ON	AT	SAVE	LETTER/S
> Rule	AS	1990	1994	even	Mar	Sun>=18	2:00s	0	-
> Rule	AS	1990	1994	odd	Mar	Sun>=1	2:00s	0	-


4) For the rules shown in "4" above, what does the "s" on the "AT time" mean? I have also seen other letters, such as "u" -- what letters are used and what is their meaning?


5) Where is the format for the Zone, Rules, Links, etc. defined and the syntax for each entry documented? (If I could have found that info, I probably would not have had to bother you with all these dumb questions!)

I GREATLY thank you for your time in answering all these newbie questions!


Sincerely,
Jon R. Kibler
Systems Architect
Year 2000 Services Manager
Jon.Kibler at aset.com

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