I’m new to this list, so please forgive me for my inexperience and lack of history. I am concerned about the timezone America/North_Dakota/Beula: Early of the local morning of 2010-11-07 the local timezone GMTOFF changed from -7:00 to -6:00, and the timezone abbreviation changed from MDT to CST. I am not questioning this historical fact. Rather I am concerned about exactly when it happened and does the tz database accurately reflect this event. Here are the relevant entries today: Zone America/North_Dakota/Beulah -6:47:07 - LMT 1883 Nov 18 12:12:53 -7:00 US M%sT 2010 Nov 7 2:00 -6:00 US C%sT Which says at 2:00am local time on 2010/Nov/07 the change from MDT to CST occurs. Local time is defined by GMTOFF == -7:00 + US timezone rules for 2010. Or in other words 2:00am + 7:00 + any daylight savings time adjustment. I’m sure I’m preaching to the choir with this audience. I think there is no dispute that: 2010-11-07 01:59:59 MDT == 2010-11-07 07:59:59 UTC At this point in time the GMTOFF == -7:00 and in the US rule SAVE == 1:00 for a total offset of -6:00. However according to US rules for this date, the local clock ticks from 01:59:59 MDT to 01:00:00 MST. To reach 2:00am local time we need to wait another hour: 2010-11-07 02:00:00 MST == 2010-11-07 09:00:00 UTC According to US time zone rules (speaking about the recognized practice in the US, not about tz database rules), there was no 2010-11-07 02:00:00 MDT. It simply did not exist. So 2010-11-07 02:00:00 local time in US rules is unambiguously a standard time, not a daylight savings time. Whereas the times [01:00:00, 01:59:59] *are* ambiguous (or more accurately described with a half-open range: [01:00:00, 02:00:00). Since 2010-11-07 02:00:00 is unambiguously a standard time, the database could read like this with no change in behavior: Zone America/North_Dakota/Beulah -6:47:07 - LMT 1883 Nov 18 12:12:53 -7:00 US M%sT 2010 Nov 7 2:00s -6:00 US C%sT But this data indicates that local time behaves as follows that morning: 2010-11-07 01:59:59 MDT = 2010-11-07 07:59:59 UTC 2010-11-07 01:00:00 MST = 2010-11-07 08:00:00 UTC 2010-11-07 01:00:01 MST = 2010-11-07 08:00:01 UTC … 2010-11-07 01:59:59 MST = 2010-11-07 08:59:59 UTC 2010-11-07 03:00:00 CST = 2010-11-07 09:00:00 UTC 2010-11-07 03:00:01 CST = 2010-11-07 09:00:01 UTC However I believe the intended behavior is: 2010-11-07 01:59:59 MDT = 2010-11-07 07:59:59 UTC 2010-11-07 02:00:00 CST = 2010-11-07 08:00:00 UTC 2010-11-07 02:00:01 CST = 2010-11-07 08:00:01 UTC I believe the tz database can be changed to give the desired behavior with: diff --git a/northamerica b/northamerica index 88423e6..aeaf12c 100644 --- a/northamerica +++ b/northamerica @@ -371,7 +371,7 @@ Zone America/North_Dakota/New_Salem -6:45:39 - LMT 1883 Nov 18 12:14:21 # of 6h47'07". Zone America/North_Dakota/Beulah -6:47:07 - LMT 1883 Nov 18 12:12:53 - -7:00 US M%sT 2010 Nov 7 2:00 + -7:00 US M%sT 2010 Nov 7 1:00s -6:00 US C%sT # US mountain time, represented by Denver If I have misinterpreted the meanings of the fields to the tz database in some way, I would appreciate an education. I’ve reread all of the man pages and other documentation I can find, and haven’t been able to rectify this contradiction on my own. If this is indeed a bug that can be corrected, I am volunteering to detect and correct similar issues elsewhere in the database and generate a pull request for https://github.com/eggert/tz. Howard