[Datetime-SIG] Adding PEP 495 support to dateutil

Tim Peters tim.peters at gmail.com
Fri Sep 18 21:00:57 CEST 2015

> ...
> I suspect dateutil has problems that are not limited to ambiguous
> datetimes in some IANA time zones.

For pytz, Stuart said he ran zdump across all zones in the database,
to drive exhaustive tests of all transition instants in every zone.
That's an excellent idea :-)

I strongly suspect dateutil will get some cases wrong simply because
it's paying attention to the gmt/std/wall indicators in tzfiles.
Those have no meaning for anything a tzinfo is trying to accomplish -
it's an "attractive nuisance" that they're even stored in a tzfile.
To convert transition times from UTC to local times (as dateutil
appears to want to do), it should simply add the current total UTC
offset, ignoring the gmt/std/wall indicators entirely.  All transition
times in tzfiles are recorded in UTC, regardless of what the
gmt/std/wall indicators say.

That won't make any difference for "most" zones because it just so
happens that the "wall" indicator is set for most transitions and the
"std" indicator is not (reflecting that most zoneinfo _source_ files
record DST transition points in local wall-clock time).  An exhaustive
test would stumble into the exceptions.  The way to fix broken cases
discovered this way is to just ignore gmt/std/wall (better, seek over
'em when reading the file - they're useless).

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