Add (partial) support for Time Zones by Reference (RFC 7809) #2051
Description
With RFC 7809, we could reduce time-zone complexity by using only the IANA timezone IDs without having to send/parse the VTIMEZONE every time.
If RFC 7809 is used, it may be safe to assume that both
- Android has up-to-date IANA TZ definitions,
- the server has up-to-date IANA tz definitions (by RFC 7809, which references the IANA definitions).
So at least in that case, we could just use the TZIDs without VTIMEZONE.
We should also re-consider our timezone strategy to always parse & use VTIMEZONE.
Maybe it provides better results to always prefer system timezones with the same name (= ignore the VTIMEZONE if a system timezone with the same name is available).
Basically this is the behavior described in the RFC (page 4):
Observation and experiments have shown that, in the vast majority of cases, CalDAV clients have typically ignored time zone definitions in data received from servers, and instead make use of their own "built-in" definitions for the corresponding time zone identifier.
We could do so and additionally add (at least partial) support for Time Zones by Reference.