Refactor RecurrencePattern.Until

[axunonb]

Remove necessity for setting the initial value to DateTime.MinValue (make it nullable)
Should RecurrencePattern.Until become a IDateTime instead of DateTilme?
Fix the method to determine the zoned date/time value

ical.net/Ical.Net/Evaluation/RecurrencePatternEvaluator.cs

Lines 30 to 34 in add58a9

	// Convert the UNTIL value to one that matches the same time information as the reference date
	if (r.Until != DateTime.MinValue)
	{
	r.Until = MatchTimeZone(referenceDate, new CalDateTime(r.Until, referenceDate.TzId)).Value;
	}

MatchTimeZone contains noop code back to v4.1.9

[axunonb]

@minichma
Here is a proposal for the refactored MatchTimeZone method.
Existing unit tests succeed when applying the fallback using reference.TzId when until.Kind != DateTimeKind.Utc.
But this is not an evidence, since unit tests were successful before, too. So I added some for all variations.

private static DateTime MatchUtilTimeZone(IDateTime reference, DateTime until)
{
    /*
       The value of the "UNTIL" rule part MUST have the same value type as the
       "DTSTART" property.  Furthermore, if the "DTSTART" property is
       specified as a date with local time, then the UNTIL rule part MUST
       also be specified as a date with local time.

       If the "DTSTART" property is specified as a date with UTC time or a date with local
       time and time zone reference, then the UNTIL rule part MUST be
       specified as a date with UTC time.
     */
    var untilCalDt = new CalDateTime(until,
        // If 'reference' is floating, then 'until' must be floating
        // If 'reference' has a timezone, 'until' MUST be UTC,
        // but in case of UTC rule violation we fall back to the 'reference' timezone
        tzId: reference.IsFloating ? null
            : (until.Kind == DateTimeKind.Utc ? CalDateTime.UtcTzId : reference.TzId),
        // 'until' must the same value type as DTSTART
        hasTime: reference.HasTime);

    // If 'reference' is floating, then 'until' is floating, too
    return reference.TzId is null
        ? untilCalDt.Value
        // convert to the reference timezone
        : untilCalDt.ToTimeZone(reference.TzId).Value;
}

[EDIT]
Here are the unit tests for all cases, with the first one failing.
Could not find out why yet. The conversion from UTC to America/New_York is correct IMHO.
Maybe you can have a look? Failed, because the test was wrong, see below.

Here are the unit test, all succeeding:

[TestFixture]
public class MatchTimeZoneTests
{
    [Test, Category("Recurrence")]
    // DTSTART with local time and time zone reference
    public void MatchTimeZone_LocalTimeWithTimeZone()
    {
        const string ical = @"
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Corp//NONSGML Event//EN
BEGIN:VEVENT
UID:example1
SUMMARY:Event with local time and time zone
DTSTART;TZID=America/New_York:20231101T090000
RRULE:FREQ=DAILY;UNTIL=20231105T130000Z
DTEND;TZID=America/New_York:20231101T100000
END:VEVENT
END:VCALENDAR";

        var calendar = Calendar.Load(ical);
        var evt = calendar.Events.First();
        var until = evt.RecurrenceRules.First().Until;

        var expectedUntil = new DateTime(2023, 11, 05, 13, 00, 00, DateTimeKind.Utc);
        Assert.That(until, Is.EqualTo(expectedUntil));

        var occurrences = evt.GetOccurrences(new CalDateTime(2023, 11, 01), new CalDateTime(2023, 11, 06));
        Assert.That(occurrences.Count, Is.EqualTo(4));
        /*
           Has 4 occurrences:
           November 1, 2023: 09:00 AM - 10:00 AM (America/New_York)
           November 2, 2023: 09:00 AM - 10:00 AM (America/New_York)
           November 3, 2023: 09:00 AM - 10:00 AM (America/New_York)
           November 4, 2023: 09:00 AM - 10:00 AM (America/New_York)
          
           November 5, 2023: 09:00 AM - 10:00 AM (America/New_York)
           must not be included, because 20231105T130000Z =>  November 5, 2023: 08:00 AM (America/New_York)
           (DST in America/New_York ended on Sunday, November 5, 2023, at 2:00 AM)
        */
    }

    [Test, Category("Recurrence")]
    // DTSTART with UTC time
    public void MatchTimeZone_UTCTime()
    {
        const string ical = @"
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Corp//NONSGML Event//EN
BEGIN:VEVENT
UID:example2
SUMMARY:Event with UTC time
DTSTART:20231101T090000Z
RRULE:FREQ=DAILY;UNTIL=20231105T090000Z
DTEND:20231101T100000Z
END:VEVENT
END:VCALENDAR";

        var calendar = Calendar.Load(ical);
        var evt = calendar.Events.First();
        var until = evt.RecurrenceRules.First().Until;

        var expectedUntil = new DateTime(2023, 11, 05, 09, 00, 00, DateTimeKind.Utc);
        Assert.That(until, Is.EqualTo(expectedUntil));

        var occurrences = evt.GetOccurrences(new CalDateTime(2023, 11, 01), new CalDateTime(2023, 11, 06));
        Assert.That(occurrences.Count, Is.EqualTo(5));
    }

    [Test, Category("Recurrence")]
    // DTSTART with floating time (no time zone)
    public void MatchTimeZone_FloatingTime()
    {
        const string ical = @"
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Corp//NONSGML Event//EN
BEGIN:VEVENT
UID:example3
SUMMARY:Event with floating time
DTSTART:20231101T090000
RRULE:FREQ=DAILY;UNTIL=20231105T090000
DTEND:20231101T100000
END:VEVENT
END:VCALENDAR";

        var calendar = Calendar.Load(ical);
        var evt = calendar.Events.First();
        var until = evt.RecurrenceRules.First().Until;

        var expectedUntil = new DateTime(2023, 11, 05, 09, 00, 00, DateTimeKind.Unspecified);
        Assert.That(until, Is.EqualTo(expectedUntil));

        var occurrences = evt.GetOccurrences(new CalDateTime(2023, 11, 01), new CalDateTime(2023, 11, 06));
        Assert.That(occurrences.Count, Is.EqualTo(5));
    }

    [Test, Category("Recurrence")]
    // DTSTART with local time and no time zone reference
    public void MatchTimeZone_LocalTimeNoTimeZone()
    {
        const string ical = @"
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Corp//NONSGML Event//EN
BEGIN:VEVENT
UID:example4
SUMMARY:Event with local time and no time zone reference
DTSTART:20231101T090000
RRULE:FREQ=DAILY;UNTIL=20231105T090000
DTEND:20231101T100000
END:VEVENT
END:VCALENDAR";

        var calendar = Calendar.Load(ical);
        var evt = calendar.Events.First();
        var until = evt.RecurrenceRules.First().Until;

        var expectedUntil = new DateTime(2023, 11, 05, 09, 00, 00, DateTimeKind.Unspecified);
        Assert.That(until, Is.EqualTo(expectedUntil));

        var occurrences = evt.GetOccurrences(new CalDateTime(2023, 11, 01), new CalDateTime(2023, 11, 06));
        Assert.That(occurrences.Count, Is.EqualTo(5));
    }

    [Test, Category("Recurrence")]
    // DTSTART with date-only value
    public void MatchTimeZone_DateOnly()
    {
        const string ical = @"
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Corp//NONSGML Event//EN
BEGIN:VEVENT
UID:example5
SUMMARY:Event with date-only value
DTSTART;VALUE=DATE:20231101
RRULE:FREQ=DAILY;UNTIL=20231105
DTEND;VALUE=DATE:20231102
END:VEVENT
END:VCALENDAR";

        var calendar = Calendar.Load(ical);
        var evt = calendar.Events.First();
        var until = evt.RecurrenceRules.First().Until;

        var expectedUntil = new DateTime(2023, 11, 05, 00, 00, 00, DateTimeKind.Unspecified);
        Assert.That(until, Is.EqualTo(expectedUntil));

        var occurrences = evt.GetOccurrences(new CalDateTime(2023, 11, 01), new CalDateTime(2023, 11, 06));
        Assert.That(occurrences.Count, Is.EqualTo(5));
    }
}

[minichma]

I think the Match method looks good. In a next step we could/should even disallow Until.Kind not matching Floating (i.e. the RFC requires (Unitl.Kind == Utc) == (tzId != null), maybe together with #653.

The tests look great too. I suggest adding a test case with a tz with negative offset, e.g. CET, because possible problems mainly originate from incorrect handling of the offset, which obviously can go in both directions.