Error with recurring events (using CalDAV calendar)

[cortesan]

Detailed Issue

I can't book any afternoons (even though I can see that the person is available), which gives me the following error: "No available users found". There's also a problem with the availability display. Whatever view you choose, from 09:00 UTC onwards, if today is Monday, all Mondays of the current month are unavailable during the times of your recurring events for the month. For example :

• person1 has recurring events of 3h30: 3 afternoons (13h30-17h UTC+1) in Tuesday, Thursday and Friday, and 1 morning (8h30-12h UTC+1) Wednesday.
  • if today is Monday, person1 is marked as unavailable every Monday of the month (because these recurring monthly events take up half the day, in the morning and afternoon)
• person2 has recurring events of 2h (15h-16h30 UTC+1 in Tuesday) and 1h30 (14h-16h30 UTC+1 in Wednesday).
  • if today is Monday, person2 is marked as unavailable only during 2h30 hours in the afternoon (14h-16h30 UTC+1)

I specify that these 2 people are available all day of Monday 24th, it's a availability display bug.

In addition, for the calendar of person1, we can see problems until April (who is the end of his recurring events). For the actual month, it's the problem described above. However, for the month of March, it's blocked on Friday (whatever day it's currently) and you can't book in the afternoons. From April, I can book on Fridays and afternoons! And for person2, it's blocked on Friday between 14h and 16h30.

This problem is also found in troubleshooting. Recurring events appear on the last day of the week view: You can see from the screenshots that if it's Tuesday, you see events on Monday, whereas if it's Wednesday, you see the same events on Tuesday. You can see a part in gray on Monday that changes according to the current day (this is what blocks Monday slots when it's Monday, etc.).

Steps to Reproduce

1. Linked cal with my CalDAV calendar
2. Check conflicts with my calendar
3. Have recurring events (set without cal)
4. Book an afternoon (on month that have recurring events)

Actual Results

• Have the following error: "No available users found" (when the person is available in this afternoon)

Expected Results

• Book an afternoon when the person is available (during months with recurring events on the CalDAV calendar)

Technical details

• Self-hosted v4.7.25 (because I have same errors that this issue #18232 for all v.4.8.*, and it's still going on for v4.9.1, v4.9.2, v4.9.3, v5.0.0 with others errors)
• node v18.20.1
• I have same errors with Firefox 135.0 and Chromium 133.0
• Recurring Event is added from SOGo webmail v5.11.2 and accepted from Thunderbird 115.18.0

Evidence

Recurring events (created without cal) are problem too: I only see the 1st occurrence on cal, after which the person is marked as available. But there is already an issue here (which I have just answered) #18060. So I accepted all previous recurring events one by one, and that doesn't fix the problem.

I don't know how cal gets the times and days of recurring events from CalDAV calendars, but it gets data that doesn't exist at certain times.

For now, some people don't check conflicts with is calendar and change their availability for each event types on cal. But this can't be a definitive solution (because we have to keep changing our availabilities). I hope it will be helpful (and hope I can use the version of the fix too).