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.)