On Tue, 31 Dec 2019 at 18:46, Paul Eggert <eggert@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