[tz] Discrepancies in time zone data interpretation at standard offset changes

Jon Skeet skeet at pobox.com
Sat Jul 11 12:06:29 UTC 2015

Heads up for Stephen Colebourne: this post highlights a discrepancy between
Joda Time and tzdump, so you may want to read carefully :)

This is in relation to my earlier message about the text format. My
confusion here is the reason I'm anxious to see canonical data :)

Currently, both Joda Time and Noda Time have some interesting code when
working out the transitions for a time zone which checks whether the later
transition occurs at the same *local* time as the earlier transition, and
if so extends the earlier transition to consume the later one.

I've worked out that this is basically to handle an oddity that sometimes
occurs when standard offsets change. One example of a zone in which this
happens is America/Juneau. Here are the zone lines from 2015e, and I've
highlighted the ones we're immediately concerned with:

Zone America/Juneau 15:02:19 - LMT 1867 Oct 18
 -8:57:41 - LMT 1900 Aug 20 12:00
 -8:00 - PST 1942
 -8:00 US P%sT 1946
 -8:00 - PST 1969
 -8:00 US P%sT 1980 Apr 27  2:00
 -9:00 US Y%sT 1980 Oct 26  2:00
* -8:00 US P%sT 1983 Oct 30  2:00*
* -9:00 US Y%sT 1983 Nov 30*
 -9:00 US AK%sT

The US rules in force at the time are:

Rule US 1967 2006 - Oct lastSun 2:00 0 S
Rule US 1976 1986 - Apr lastSun 2:00 1:00 D

So, the first bolded zone line comes to an end at the instant of the
transition into standard time, while standard time is -8 (so a wall offset
of -7). I assume this transition should *not* be included in the eventual
set of transitions (i.e. the bound given is exclusive). This would occur
at 1983-10-30T09:00Z.

Now, in the second bolded zone line, the standard offset is -9 instead, so
the transition into daylight saving time would occur at 1983-10-30T10:00Z.

So looking at the rules in a naive way, residents of Juneau would have to
change their watches twice on October 30th 1983. At 2am they'd put their
watches back an hour to account for the change in standard time (but still
be in daylight saving time)... and then when it next hit 2am they'd put
their watches back *another* hour to account for the transition into
standard time. Now, I'm pretty sure that's not what's intended, and indeed
zdump shows just a single transition setting local time back two hours at

America/Juneau  Sun Oct 30 08:59:59 1983 UTC = Sun Oct 30 01:59:59 1983 PDT
America/Juneau  Sun Oct 30 09:00:00 1983 UTC = Sun Oct 30 00:00:00 1983 YST

... whereas Noda Time (and, I've just checked, Joda Time) have a single
transition at 1983-10-30T10:00Z instead.

Is it documented somewhere what should happen in this case? (As a side
question, is there a single canonical place documenting the time zone data
format? I've found various sources over the years, but it would be nice to
have somewhere canonical - ideally either within each release or on the
IANA home page. The Theory file gives a lot of interesting more
abstract information,
but a single place saying "Here is what each file in the release means,
with details of its format" would be very welcome. Apologies if it already
exists and I've just missed it...)

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