ics export EXDATE invalid date-time

[mujo-hash]

Checklist

• I can reproduce the bug with the latest version given here.
• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one bug.
• I have read and understood the contribution guidelines.

Affected app version

1.10.3

Affected Android/Custom ROM version

Android 16 / GrapheneOS 2026030201

Affected device model

Pixel 7

How did you install the app?

F-Droid / IzzyOnDroid

Which calendar service or provider do you use, if any?

No response

Which sync adapter app do you use for calendar synchronization, if any?

No response

Steps to reproduce the bug

1. Menu > Settings > Export to ics file
2. included export events, export tasks, export part entries too, selected one calendar
3. Attempted to import the ics file via radicale's web interface.

Expected behavior

I expected that the fossify exported ics file would import successfully in radicale.

Actual behavior

Noted the error on the interface and the logs stating: '20250429' is not a valid DATE-TIME at the line with EXDATE.

I then opened an issue on radicale, assuming it was an issue with their import Kozea/Radicale#2008 but per the reading of the rfc https://icalendar.org/iCalendar-RFC-5545/3-8-5-1-exception-date-times.html it seems that the exported ics file might be invalid.

My novice reading of the rfc is that EXDATE can either be in the form EXDATE;VALUE=DATE:20250429, EXDATE;VALUE=DATE-TIME:19960402T010000Z, or EXDATE:19960402T010000Z but I'm not 100% confident.

After taking the exported ics and removing unrelated events, here is the minimal file:

 BEGIN:VCALENDAR
 PRODID:-//Fossify//NONSGML Event Calendar//EN
 VERSION:2.0
 BEGIN:VEVENT
 SUMMARY:Chess ✨
 UID:540b1f50c0674021ba838e3d602c0fb41742356467766
 X-FOSSIFY-CATEGORY-COLOR:-16434009
 CATEGORIES:Chess wizard
 LAST-MODIFIED:20251205T074226Z
 TRANSP:OPAQUE
 DTSTART:20250401T210000Z
 DTEND:20250401T210000Z
 X-FOSSIFY-MISSING-YEAR:0
 DTSTAMP:20260302T055208Z
 CLASS:PUBLIC
 STATUS:CONFIRMED
 RRULE:FREQ=WEEKLY;INTERVAL=1;COUNT=9;BYDAY=TU
 EXDATE:20250429
 END:VEVENT
 END:VCALENDAR

Using the icalendar validator https://icalendar.org/validator.html states that the file is valid so we attempted to submit an issue with their feedback form but it failed.

Screenshots/Screen recordings

No response

Additional information

No response

[mujo-hash]

seems like maybe just fillIgnoredOccurrences in app/src/main/kotlin/org/fossify/calendar/helpers/IcsExporter.kt needs to change?

Assuming events and tasks both need the EXDATE format to change