bug#56125: 28.1; Timezone conversion sometimes wrong for recurring events, when importing ical to diary

[Martin Apel via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Hi Lars,


Lars Ingebrigtsen <larsi@gnus.org> writes:

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> Martin Apel <martin.apel@3ds.com> writes:
>
>> When importing the attached minimal ical file using
>> icalendar-import-file, the resulting time is off by one hour. The
>> problem probably lies somewhere in
>> icalendar--convert-recurring-to-diary, but I couldn't pinpoint the
>> exact location.  The only meeting in the attached icalendar file is
>> defined in timezone 'America/New_York' at 10 am. It shows up in my
>> diary (which is timezone 'Europe/Berlin') at 3pm, even though the time
>> difference between the two timezones is 6 hours.
>
> No, the difference varies -- Daylight Savings started on March 13 in the
> US but on March 27 in the EU, and in that period there was a five hour
> difference.
>
> And:
>
> DTSTART;TZID="America/New_York":20220317T100000
> DTEND;TZID="America/New_York":20220317T103000
>
> This was in the middle of that period.  So this seems like it's working
> as it should?
>
> But if I'm reading the ical file right, this is a recurring (weekly?)
> event, so it should be at 4pm after March 27...

I thought that it might have something to do with the different start dates for daylight savings time. However it should
show up at 4pm, but it shows up at 3pm after March 27.

Maybe this is a more general problem, that the diary does not consider timezones at all. I think, that the conversion
functionality would need to convert each date of a recurring event separately, as the daylight savings time might have
changed between consecutive repetitions.  However the diary syntax does not allow to specify this, as far as I
understand. So maybe this is more of a shortcoming in the diary syntax than a conversion problem.  Probably instead of
diary-cyclic another sexp would be needed, which also takes the time of day into account, which diary-cyclic does not
do.

Thanks,

Martin
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