Recurring event with exclusions produce warnings

[varac]

Describe the bug
I have en recurring event with many exclusions and it seems khal produces a warning for every exclusion:

warning: In event d215d83c-101b-4116-aed8-d481b8cdc47e.ics, excluded instance starting at 2025-06-05 09:30:00+02:00 not found, event might be invalid.
warning: In event d215d83c-101b-4116-aed8-d481b8cdc47e.ics, excluded instance starting at 2025-06-12 09:30:00+02:00 not found, event might be invalid.
...

To Reproduce

Use khal with the attached .ics file.

Expected behavior
If the .ics is invalid, the warning should be more detailed, so users understand what's the problem.
If the .ics is valid, khal should not produce a warning.

OS, version, khal version and how you installed it:

• The output of khal --version: khal, version 0.13.0
• Installation method: Manjaro Arch package
• python version: 3.13.7
• OS: Arch (Manjaro)

d215d83c-101b-4116-aed8-d481b8cdc47e.ics.txt

[WhyNotHugo]

The error implies that those "excluded dates" from your recurring event are not valid instances of the recurring event. For example: an event says "repeat every Monday at 12:00" but has an exception of the style "except a specific Thursday at 19:00".

This particular event:

• Starts on 2025-09-04 (Thursday at 09:30)
• Repeats weely every Thursday.

It has two exceptions to the rule:

• 20250605T093000 = 2025-06-05 (Thursday)
• 20250612T093000 = 2025-06-12 (Thursday)

These exceptions are not valid (they're before the first date).

I guess we could have a longer explanation in the documentation and include a reference in this error.

[varac]

Thanks for the explanation. I guess it's Thunderbird's fault to modify the event like this.
And yes, it would be nice to reference the updated docs.