Discussion: iCalendar invites: UID, SEQUENCE and getting time zones right
Entries
'Use UTC for one-off events with attendees in many zones' picks the time form by the wrong property. What decides between UTC and zone-anchored local time is what the event is anchored to, not whether it recurs. A one-off product launch at a fixed global instant is a UTC event. A one-off board meeting on a given date at 09:00 Zurich time is anchored to a local wall clock, and if the zone's rules change between scheduling and the date (governments do this on short notice: Mexico abolished DST in 2022, Egypt reinstated it in 2023), the UTC form silently moves the meeting while the `TZID` form keeps it at 09:00. Multi-zone attendance does not change that; each attendee's client converts either form. The rule should read: anchor to a `TZID` whenever a human decided the time on a local clock, use UTC when the instant itself is the fact, and treat recurrence as irrelevant to the choice.
On 'zone names that are not IANA identifiers': RFC 5545 does not require IANA names, the `TZID` is any text identifier resolved against the embedded `VTIMEZONE`, and Microsoft Outlook and Exchange emit Windows zone names such as `W. Europe Standard Time`. A consumer that maps `TZID` to a zone database therefore needs the CLDR `windowsZones.xml` table, which maps each Windows name to IANA zones per territory, and must fall back to the embedded `VTIMEZONE` rules when the name is unknown, which is exactly why the RFC requires the component. On the Python side, `zoneinfo` (3.9) supplies IANA rules from the system database or the `tzdata` package, and the `icalendar` package generates `VTIMEZONE` components from them. For the RFC 5546 side, a `REPLY` must carry only the replying `ATTENDEE` with the same `UID`, and the organiser applies it only if the `SEQUENCE` matches its current revision, so a reply to a superseded invite is correctly ignored rather than merged.
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