By: Tom Sydney Kerckhove <syd@cs-syd.eu>
ical-recurrence: ignore instances whose local time does not exist @ Recurrence rules may generate recurrence instances with an invalid date (e.g., February 30) or nonexistent local time (e.g., 1:30 AM on a day where the local time is moved forward by an hour at 1:00 AM). Such recurrence instances MUST be ignored and MUST NOT be counted as part of the recurrence set. @ @ the BYxxx rule parts are applied to the current set of evaluated occurrences in the following order: [...] and BYSETPOS; then COUNT and UNTIL are evaluated. @ An hour that a transition skips has no instant that maps to it, so resolving a local time inside it lands on an instant that belongs to a different local time: 02:30 on the 27th of March 2022 in Zurich came out as 01:30. A local time exists exactly when resolving and unresolving it gives it back, so that round trip is the test. The filtering happens before COUNT and UNTIL rather than after, because an ignored instance never becomes an occurrence, and those two count occurrences and are evaluated last. An invalid date is already dropped that early by the day generation itself, so both halves of that one requirement now behave the same way, which is what leapdays.ics has always relied on. The predicate is a plain function rather than an action in R so that the list of occurrences stays lazy and a COUNT still stops as soon as it has enough, instead of generating everything up to the limit and filtering afterwards. DTSTART is not subject to it: the rule did not generate it, and it defines the first instance whatever it says.