[tz] Possible bug in the tz database

Paul Eggert eggert at cs.ucla.edu
Sun Jun 7 02:05:56 UTC 2015

Howard Hinnant wrote:
> I would appreciate an education.

The intent is that the UNTIL column (which specifies a transition time) is 
interpreted as of the rules in effect before the transition.  In the case you've 
identified, two transitions occur simultaneously.  One is that DST is no longer 
in effect; the other, that the location moves from mountain to central time.  In 
this case, the UNTIL column is interpreted without either change in effect, 
i.e., in mountain daylight time.  Under this interpretation, the tz database is 
already correct and the patch you proposed would not change its behavior.

> 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.

Actually, I think the common practice in the US is to say that the clock moves 
backward from 02:00 to 01:00 local time, i.e., 02:00 and 01:00 are both observed 
twice on fall-back days.  The tz database takes a stricter approach though, an 
approach derived from common computer practice: it says that 01:00 occurs twice 
but 02:00 occurs only once in generated time stamps.  However, this approach 
does not apply to the UNTIL column, which commonly refers to a local time like 
02:00 even when the computer-generated time stamps jump from 01:59.999...9 back 
to 01:00 or forward to 03:00 and never hit 02:00 exactly.

Attached is a proposed patch to the documentation to try to make this a bit clearer.
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