BUG: yearly recurring event on last sunday of month X is shown every sunday

[Starkstromkonsument]

I'm using Nextcloud Calendar with the sync client DAVx on my andorid devices and Etar. I have two events in my calandar, that should remind me of the clock change for the daylight saving time.

Etar shows the events on every sunday in march/october, not only on the last sunday as intended. It works fine in the WebUI of Nextcloud Calendar, Thunderbird Calendar and Samsung Calendar.

last sunday every march

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//IDN nextcloud.com//Calendar app 2.1.2//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
CREATED:20201025T160620Z
DTSTAMP:20201025T160822Z
LAST-MODIFIED:20201025T160822Z
SEQUENCE:2
UID:3b52ba5b-954d-4f83-a902-e43d1ab53cd7
DTSTART;VALUE=DATE:20200329
DTEND;VALUE=DATE:20200330
SUMMARY:Zeitumstellung 2:00 auf 3:00
DESCRIPTION:Umstellung von Winterzeit auf Sommerzeit. Jedes Jahr am letzten
  Wochenende des März (in der Nacht von Samstag auf Sonntag).
TRANSP:TRANSPARENT
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=SU;BYSETPOS=-1
END:VEVENT
END:VCALENDAR

last sunday every october

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//IDN nextcloud.com//Calendar app 2.1.2//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
CREATED:20201025T155842Z
DTSTAMP:20201025T160421Z
LAST-MODIFIED:20201025T160421Z
SEQUENCE:2
UID:40d997d7-0729-4381-b993-5eec31ef70b9
DTSTART;VALUE=DATE:20201025
DTEND;VALUE=DATE:20201026
SUMMARY:Zeitumstellung 3:00 auf 2:00
DESCRIPTION:Umstellung von Sommerzeit auf Winterzeit. Jedes Jahr am letzten
  Wochenende des Oktobers (in der Nacht von Samstag auf Sonntag)
TRANSP:TRANSPARENT
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=SU;BYSETPOS=-1
END:VEVENT
END:VCALENDAR

[tchernobog]

I see a similar problem with an event like the following:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//IDN montecristosoftware.eu//cooking-schedule 0.0.0//EN
BEGIN:VEVENT
UID:[email protected]
DTSTAMP:20220729T192029Z
SUMMARY:Pasta fredda con pesto senza aglio
DESCRIPTION:cookbook@72130
LOCATION:https://ricette.giallozafferano.it/Pasta-fredda-con-pesto-senz-agl
 io.html
DTSTART;TZID=Europe/Berlin:20220705T184000
DTEND;TZID=Europe/Berlin:20220705T190000
RRULE:FREQ=YEARLY;BYDAY=TU;BYWEEKNO=27
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER;RELATED=START:-PT15M
DESCRIPTION:Pasta fredda con pesto senza aglio
END:VALARM
END:VEVENT
END:VCALENDAR

I can confirm this bug, it applies also to RRULE:FREQ=YEARLY;BYDAY=<day>;BYWEEKNO=<weekno>.

[toolstack]

I've tracked this down to a bug in the Android calendar code, it basically ignores BYWEEKNO and BYSETPOS in RRULE's.

I've submitted PR #1465 to resolve by using a different RRULE processor to double check if the ones returned by Android are valid or not.