9adc0d2f

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

ical-recurrence: exclude exception dates by instant as well as by spelling

[section 3.8.5.1](https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.5.1)

@
Value Type:  The default value type for this property is DATE-TIME.
   The value type can be set to DATE.
@

@
Property Parameters:  IANA, non-standard, value data type, and time
   zone identifier property parameters can be specified on this
   property.
@

@
The final recurrence set is generated by gathering all of the
start DATE-TIME values generated by any of the specified "RRULE"
and "RDATE" properties, and then excluding any start DATE-TIME
values specified by "EXDATE" properties.
@

A time zone identifier is allowed on an EXDATE and nothing ties it to the
one DTSTART uses, so a conforming exception can name an instance in UTC,
or in another time zone, while the instance itself is written in the time
zone of DTSTART.  Comparing the values as written misses all of those.

Compare what they resolve to.  This runs on top of the comparison of the
written values rather than replacing it, so an exception or an occurrence
whose time zone the calendar does not define is still excluded on the
strength of its spelling.  Nothing that used to be excluded stops being
excluded, and nothing starts failing for want of a time zone, which
matters because removeExceptionDatetimesSet runs for every event.

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