EXDATE with comma-separated values on a single line silently drops to empty list (RDATE works correctly)

[kipcole9]

Environment

• ical from github: "expothecary/ical" (main branch)
• Elixir 1.20.0-rc.4-otp-28
• Tested via ICal.from_ics/1

Summary

RFC 5545 §3.8.5.1 (EXDATE) and §3.8.5.2 (RDATE) both permit multiple values either as separate properties OR as one property with comma-separated values. Per the RFC grammar:

exdate     = "EXDATE" exdtparam ":" exdtval *("," exdtval) CRLF
rdate      = "RDATE" rdtparam ":" rdtval *("," rdtval) CRLF

ical handles comma-separated values correctly for RDATE but silently parses EXDATE as an empty list. This is an asymmetry — both fields have identical RFC syntax.

Minimal reproduction

ics_exdate = """
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//T//EN
BEGIN:VEVENT
UID:m
DTSTAMP:20220101T000000Z
DTSTART:20220601T090000Z
DTEND:20220601T100000Z
RRULE:FREQ=WEEKLY;COUNT=4
EXDATE:20220608T090000Z,20220622T090000Z
END:VEVENT
END:VCALENDAR
"""

ics_rdate = """
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//T//EN
BEGIN:VEVENT
UID:m
DTSTAMP:20220101T000000Z
DTSTART:20220601T090000Z
DTEND:20220601T100000Z
RRULE:FREQ=WEEKLY;COUNT=1
RDATE:20220615T140000Z,20220620T100000Z
END:VEVENT
END:VCALENDAR
"""

%ICal{events: [e_ex]} = ICal.from_ics(ics_exdate)
%ICal{events: [e_rd]} = ICal.from_ics(ics_rdate)

IO.inspect(e_ex.exdates, label: "exdates")
IO.inspect(e_rd.rdates, label: "rdates")

Actual restult

exdates: []
rdates: [~U[2022-06-15 14:00:00Z], ~U[2022-06-20 10:00:00Z]]

Expected result

exdates: [~U[2022-06-08 09:00:00Z], ~U[2022-06-22 09:00:00Z]]
rdates:  [~U[2022-06-15 14:00:00Z], ~U[2022-06-20 10:00:00Z]]

Workaround

Using one EXDATE property per value works as expected:

EXDATE:20220608T090000Z
EXDATE:20220622T090000Z

This parses correctly into the expected 2-element list.

Impact

EXDATE:d1,d2,… is a common form in real-world .ics exports (including Google Calendar and Apple Calendar). Events using it silently lose their exclusions — the calendar renders "extra" occurrences that should have been removed.

Suspected location

Whatever handler splits RDATE's comma-separated values isn't being invoked for EXDATE. Likely a missing String.split(",") (or equivalent) in the EXDATE parse path, while the RDATE path has it.