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

Tim Parenti tim at timtimeonline.com
Tue Mar 12 03:35:50 UTC 2019

On Mon, 11 Mar 2019 at 18:30, Paul Eggert <eggert at cs.ucla.edu> wrote:

> On 3/11/19 4:18 AM, Christopher Wong wrote:
> > Is it correct that the switch between LMT to GMT is set to the specified
> start of the time range?
> Yes. If you tell zic's new -r option to discard info for timestamps
> before time T, then the first transition in the output TZif file will be
> at time T. This is so that a TZif reader can determine the range of
> timestamps that the TZif file covers. See Internet RFC 8536 section 5.1.

Yes, but in the examples given which truncate from 1977 and 1984, that
shouldn't be a transition from LMT, as LMT was not observed in that Zone
since 1912: "As with untruncated TZif files, time type 0 indicates local
time immediately before the start point, and the time type of the first
transition indicates local time thereafter."

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).  In each of these examples,
that's GMT, not LMT.  The file would then have the required transition at
time N, though in these cases, that would be a no-op transition to the same
time type, as it's still GMT.

Tim Parenti
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