#227 emitted TZID with the IANA name and nothing defining it, on the reasoning
that every client resolves those names and that generating a definition would
mean shipping a zone database. Both halves were wrong.
Measured, not assumed. Run an export through ical.js -- Mozilla's own
iCalendar library, the one Thunderbird's calendar uses -- and a TZID with no
VTIMEZONE beside it does not resolve: it falls back to floating time. A 09:00
in Phoenix then reads as 09:00 wherever the file is opened, seven hours out,
silently, on every timed event in every export.
as exported | zone: floating | UTC: 09:00Z
with a VTIMEZONE added | zone: America/Phoenix | UTC: 16:00Z
The database was already here, too. The browser has IANA behind Intl, and an
offset for an instant is a formatting question: format the instant into the
zone, read the clock back, and the difference is the offset. Transitions are
found by walking month by month for the ones where the answer changes and
bisecting inside them -- no rules are known, so none can be got wrong.
Each transition is its own dated sub-component rather than an RRULE. More
lines and no cleverness: a derived rule that is subtly wrong moves somebody's
meeting, while a list of dates can only be incomplete at its ends, which is
what the window is for -- the year before the earliest event to ten years past
the latest, an open-ended weekly meeting being the case that needs it.
A zone Intl does not know is left undefined rather than described from
nothing; the TZID stays on the event, which is where it was. TZNAME is dropped
where Intl offers "GMT+9", which only repeats the offset beside it.
Confirmed the same way it was found. Berlin now resolves to +0200 in September
and +0100 in December, so the transitions are being applied and not just an
offset.
Refs #216.
jcoffey-dev
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