RFC 5545, "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", September 2009

Note: This RFC has been updated by RFC 5546, RFC 6868, RFC 7529, RFC 7953, RFC 7986, RFC 9073, RFC 9074, RFC 9253

Errata ID: 4271
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Neil Jenkins
Date Reported: 2015-02-12
Verifier Name: Barry Leiba
Date Verified: 2015-02-17

Section 3.3.10 says:

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.

It should say:

Recurrence rules may generate recurrence instances with an invalid
date (e.g., February 30). Such recurrence instances MUST be ignored
and MUST NOT be counted as part of the recurrence set.

Recurrence rules may generate recurrence instances with a 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 are
handled as specified in Section 3.3.5.

Notes:

The section about ignoring times that don't occur in the timezone of the expansion is contradicted by the later passage (p44):

"""If the computed local start time of a recurrence instance does not
exist, or occurs more than once, for the specified time zone, the
time of the recurrence instance is interpreted in the same manner
as an explicit DATE-TIME value describing that date and time, as
specified in Section 3.3.5."""

And this is the behaviour implemented by major clients such as Calendar.app and Google Calendar.

Report New Errata