fix(caldav): Fix reminder timezone drift for all-day events

[ChristophWurst]

Contributes to nextcloud/calendar#4778

Summary

All day events contain no timezone information. Yet reminders are specified relative to the event start. Each calendar has an optional timezone. If it's set, we should use that to determine the correct UTC timestamp for the reminder. As a fallback we should use UTC.

Ref https://www.rfc-editor.org/rfc/rfc4791#section-5.2.2

 The CALDAV:calendar-timezone property is used to
specify the time zone the server should rely on to resolve "date"
values and "date with local time" values (i.e., floating time) to
"date with UTC time" values. The server will require this
information to determine if a calendar component scheduled with
"date" values or "date with local time" values overlaps a CALDAV:
time-range specified in a CALDAV:calendar-query REPORT.

TODO

• Test all day events when calendar has timezone set
• Test all day events when calendar does not have timezone set
• Test event when calendar has timezone set
• Test event when calendar does not have timezone set

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[ChristophWurst]

/backport to stable25

[ChristophWurst]

/backport to stable24

[miaulalala]

Looks good

[st3iny]

Tested and works. Nice one!

Setting the calendar timezone is not possible from our UI yet, right?

[ChristophWurst]

Setting the calendar timezone is not possible from our UI yet, right?

Correct. Coming with nextcloud/calendar#4935 hopefully

[ChristophWurst]

/backport to stable24

[Luncheon3462]

I’m just an end user, not a dev but running NC 26.0.3. The time zone drift for all day event reminders still occurs for me. Doesn’t matter if I use the personal calendar that is automatically created or a new calendar that I create. My time zone is set correctly in the nc calendar app settings app. Is this expected still?

[st3iny]

@Luncheon3462 That is unfortunate. Please open a new issue and follow the bug template so that we can track and investigate the issue.

[Luncheon3462]

@st3iny I'll do my best. Not a developer or github user. Only created an account to let you, @ChristophWurst@tcitworld@miaulalala know that the bug may still be present. I also posted on the nextcloud forum, and at least one other person is reporting the same. That thread is at https://help.nextcloud.com/t/calendar-timezone-bug-36192/167212

[Luncheon3462]

@st3iny i did my best. see report at #39640.