Question about Zone/Rule semantics

[Damon Chaplin]

I have a question about how to interpret Zone/Rule data.
I'm unsure about this zone:


Zone America/Chihuahua	-7:04:20 -	LMT	1921 Dec 31 23:55:40
			-7:00	-	MST	1927 Jun 10 23:00
			-6:00	-	CST	1930 Nov 15
			-7:00	-	MST	1931 May  1 23:00
			-6:00	-	CST	1931 Oct
			-7:00	-	MST	1932 Apr  1
			-6:00	-	CST	1996
			-6:00	Mexico	C%sT	1998
			-6:00	-	CST	1998 Apr Sun>=1 3:00
			-7:00	Mexico	M%sT


The corresponding Rule lines are:

Rule	Mexico	1996	2000	-	Apr	Sun>=1	2:00	1:00	D
Rule	Mexico	1996	2000	-	Oct	lastSun	2:00	0	S


What is the wall-clock offset from UTC on April 10th 1998?


The last UNTIL value is "1998 Apr Sun>=1 3:00", which is just after the April Rule
came into effect ("Apr	Sun>=1	2:00"). So I'm unsure if the Rule is supposed to
still be in effect or not. i.e. I don't know if it should be -7:00 (standard time)
or -6:00 (daylight time).

It makes more sense if it is daylight time, but then why didn't the UNTIL value
just use "Apr Sun>=1 2:00" and make it obvious.


My general question is: do Rules apply to a particular period within a Zone
(i.e. one line in the Zone description) even when they happened before the Zone
period started?


Damon

(I'm working on a program to convert the timezone database to the format
needed by the iCalendar specification, for use by calendaring apps etc.)