cc79e0a2

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

ical-recurrence: add unresolveTimestampR to render a Timestamp in a display timezone

resolveDateTime turns a UTC/zoned DateTime into an absolute TimestampUTCTime.
Consumers that want to show that instant as a human-readable local time must
convert it back into a display timezone's wall-clock -- and doing that by hand
(keeping the UTC wall-clock) is wrong by the zone's offset (1-2h). This adds the
DST-correct helper so consumers stop hand-rolling it.

Regression test encodes the concrete case that was mis-handled downstream: a
19:00Z instant in a +02:00 (CEST) zone renders as 21:00 local, not 19:00.

Timeline