[tz] [PROPOSED] zic option for including data from a certain year onward

Tim Parenti tim at timtimeonline.com
Fri Mar 15 00:31:05 UTC 2019

On Thu, 14 Mar 2019 at 12:26, Paul Eggert <eggert at cs.ucla.edu> wrote:

> On 3/11/19 8:35 PM, Tim Parenti wrote:
> > So if my truncation range starts with POSIX timestamp N, I would
> > expect time type zero to correctly and accurately represent timestamp
> > N−1 (though, of course, not necessarily N−2 or any earlier).
> Thanks for catching that bug. I installed the attached patch.

Thanks, though I should note I have a quibble with a small assertion in
your patch notes:

It’s nicer (though not required by RFC 8536) that when the start of a TZif
> file is truncated, the default time type is that of timestamps just before
> the first transition.

Section 5.1 of RFC 8536 requires that "[a]ll represented information that
falls inside the truncation range MUST be the same as that represented by a
corresponding untruncated TZif file."  By not making the default time type
that of timestamps just before the range starts at time N, this creates a
false and inaccurate transition at time N.  By ensuring that timestamp N−1
is accurately represented, either a correct transition is inserted at time
N (if a transition does indeed normally exist at time N), or a
fictitious-though-accurate no-op transition is inserted at time N.  Either
way, the data representing the transition at time N would then be correct.

Therefore, it is certainly my understanding that such behavior is indeed
required by RFC 8536.  And yes, that is very much because it's nicer.

Tim Parenti
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