[tz] Hour 26? (was: Chaning time zone for Unix based tasks)

[Tim Parenti]

On Tue, 31 Dec 2019 at 18:46, Paul Eggert <eggert at cs.ucla.edu> wrote:

> Please see Internet RFC 8536 section 3.3.1 and
> <http://austingroupbugs.net/view.php?id=1252>.


RFC 8536 §3.3.1 extensions state only "[t]he hours part of the transition
times may be signed and range from -167 through 167", but does not itself
define the behavior if minutes are specified.  Of course, this is not a
major concern when the hours are positive (or 0), as the intent is fairly
straightforward, and the current POSIX definitions being extended imply as
much by stating that "[t]he time [of transition] has the same format as
[the] offset", just without sign.

As it happens, Pacific/Chatham is currently the only zone that must specify
minutes in its TZ string, since its DST transitions are linked to New
Zealand's main islands and therefore don't take place at the top of the
hour, local time:

"<+1245>-12:45<+1345>,M9.5.0/2:45,M4.1.0/3:45"

For negative hours, Austin Group issue 0001252 recommends that '-3:30'
stands for 20:30 the previous day, but this may not be the most instructive
example.  If, for example, New Zealand were to change its spring transition
date from "last Sunday in September" to, instead, "Sunday preceding last
Monday in September" (and similarly for fall in April), this would result
in:

"<+1245>-12:45<+1345>,M9.5.1/-21:15,M4.1.1/-20:15"

While this logically follows how we tend to think of "negative time" (i.e.,
as a "countdown" that negates the entire expressed time interval), it may
not be obvious to the casual observer that "-21:15" and "-20:15" refer to
local times that are nominally at "45 past the hour".  Since this affects
how one should approach parsing, perhaps an example not on the half-hour
might make that intent a bit clearer.

--
Tim Parenti
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