[tz] Precise meaning of FROM and TO in a rule

Jon Skeet skeet at pobox.com
Tue Jul 8 06:00:56 UTC 2014

Thanks for the fabulous example of Godthab.

I've confirmed that http://www.timeanddate.com/time/zone/greenland/nuuk
<http://timeanddate.com>, Noda Time and Java 8 all put the next transition
at 2014-10-26T01:00:00Z, so 1am Sunday UTC, 11pm *Saturday *local time. So
that's the first thing that should be clarified in the documentation, IMO -
assuming that we're happy that *is* the correct behaviour.

The next part - and where I started - is which year that transition would
be assumed to be in. Sure, for October it's not a problem... but imagine
the rule had been for the first Sunday in January, with a rule starting in
2012. When would the rule have first applied? Either it wouldn't have
applied in 2012 at all, or it would have applied in *UTC* 2012, which was
*local* 2011 at the point of the transition. Again, I realize this is
mostly theoretical - if everyone's happy to agree that we just avoid
getting to this edge condition (through careful rule crafting), that'd be
great... but should also be documented, I think.


On 8 July 2014 01:15, Tim Parenti <tim at timtimeonline.com> wrote:

> On 7 July 2014 18:44, Jon Skeet <skeet at pobox.com> wrote:
>> I think at the moment I always assume that the time of day and the date
>> are in the same frame of reference - so in the above example, I suspect I'd
>> find a transition at Monday April 1st, 2am UTC
>  This, too, is my personal understanding of at least the *intent* behind
> the interpretation of years, months, and dates in FROM, TO, TYPE, IN, and
> ON,  That is, they are to take the same interpretation as the suffix which
> applies to AT.
> In your example: Since 2:00u is specified for the transition, Apr Mon>=1
> should be treated as Universal time, then find 02:00Z within that day.  So
> transition Monday 1 April 02:00 UTC, which is Sunday 31 March 18:00 UTC–8.
> On 7 July 2014 17:57, Paul Eggert <eggert at cs.ucla.edu> wrote:
>> Transitions are given in terms of the local time before the transition,
>> as otherwise a transition could be self-referential.
> This doesn't seem it should apply to transition rules with "u"/"g"/"z"
> suffix; only those with "s" or "w"/no suffix.
> "Standard time" rules would be computed similarly to "Universal time"
> rules.  Find the span of the specified date in the zone's standard time (as
> reckoned by the offset PRIOR to the transition), then find the specified
> standard time within that span.
> "Wall clock time" rules with no suffix (or a "w" suffix) would find the
> span of the date in the zone's wall clock time (as reckoned by the PRIOR
> offset), then find the proper wall clock time.
> I think this seems the most logical for maintenance and rule-writing, as
> any rule specified in, e.g., UTC, ought to recur in UTC.
> For a concrete example, see how current EU rules are meant to apply to
> America/Godthab.  Transitions occur at Mar lastSun 1:00u and Oct lastSun
> 1:00u, which should be equivalent to Mar Sat>=24 22:00s and Oct Sat>=24
> 22:00s.
> --
> Tim Parenti
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