The :dst zone model assumed northern ordering (dst-start < dst-end, DST =
[start, end)). Southern zones — DST begins ~Oct and ends ~Apr — have
dst-start > dst-end, so the old (>= start AND < end) test was never true and
ev-tz-offset returned the standard offset year-round.
Fix: detect the ordering. start < end → DST is [start, end); start > end →
DST wraps the calendar-year boundary, active when (utc >= start OR utc < end).
Add predefined ev-tz-sydney (AEST +600 / AEDT +660; transitions 02:00 AEST
first-Sun-Oct and 03:00 AEDT first-Sun-Apr, both 16:00 UTC the preceding
Saturday → rule time -480). VTIMEZONE export is already rule-agnostic, so
southern zones round-trip through iCal unchanged (the -480 folds the
from-offset back to the correct local 02:00/03:00 DTSTART).
+8 timezone tests (now 25): summer/winter offsets, both transition dates,
local->utc in both seasons, and a daily expansion crossing the autumn DST-end
that shifts in UTC (1320,1320,1380,1380,1380) while staying 09:00 local.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
giles
78b45a331e