Outlook not parsing ics invites with rrule BYMONTHDAY=-1 or BYSETPOS=-1 properly
Current Behavior When an ics event is attached or imported as a file, and the rrule contains either BYMONTHDAY=-1 or BYSETPOS=-1, it is not properly converted to the outlook/exchange meeting type and the ics attachment will read as "not supported calendar message". It is possible to work around this by appending an additional semicolon to any property in the rrule string prior to the BYMONTHDAY=-1 or BYSETPOS=-1. However, the workaround above is actually syntactically invalid. Sending .ics invites that contain an rrule with this workaround that are opened in any application other than outlook would fail to be processed. Reproduction environment MacOS Sequoia 15.3.2 (24D81). definitely doesn't matter. using outlook for mac desktop app, outlook web app Reproduction steps See the attached ics invites - note the only difference is the additional semicolon before BYMONTHDAY in the second invite. Save these as files with the .ics extension, and then open them with outlook. Invalid invite - an event that repeats monthly on the last day of the month starting on May 31st for two months. BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:adamgibbons/ics METHOD:REQUEST X-PUBLISHED-TTL:PT1H BEGIN:VEVENT UID:726df22a-cc8d-4276-9be9-236f4af16450 SUMMARY:Reserved by user DTSTAMP:20260413T225217Z DTSTART:20260531T050000Z DTEND:20260531T060000Z SEQUENCE:0 DESCRIPTION:hi LOCATION:hi STATUS:CONFIRMED CATEGORIES: TRANSP:TRANSPARENT X-MICROSOFT-CDO-BUSYSTATUS:FREE ATTENDEE;ROLE="REQ-PARTICIPANT";CN="user":mailto:user@example .com RRULE:FREQ=MONTHLY;INTERVAL=1;COUNT=2;DTSTART=20260531T050000 Z;BYMONTHDAY=-1; END:VEVENT END:VCALENDAR Valid invite - same event but with an additional ; before BYMONTHDAY: BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:adamgibbons/ics METHOD:REQUEST X-PUBLISHED-TTL:PT1H BEGIN:VEVENT UID:726df22a-cc8d-4276-9be9-236f4af16450 SUMMARY:Reserved by user DTSTAMP:20260413T225217Z DTSTART:20260531T050000Z DTEND:20260531T060000Z SEQUENCE:0 DESCRIPTION:hi LOCATION:hi STATUS:CONFIRMED CATEGORIES: TRANSP:TRANSPARENT X-MICROSOFT-CDO-BUSYSTATUS:FREE ATTENDEE;ROLE="REQ-PARTICIPANT";CN="user":mailto:user@example .com RRULE:FREQ=MONTHLY;INTERVAL=1;COUNT=2;DTSTART=20260531T050000 Z;;BYMONTHDAY=-1; END:VEVENT END:VCALENDAR