RecurrencePatternEvaluator treats DTSTART and UNTIL timezones incorrectly

[minichma]

EventEvaluator and RecurrencePatternEvaluator seem to handle DTSTART and UNTIL's timezones incorrectly. It seems, that

• RecurrencePatternEvaluator doesn't consider the time zone of the start value passed into RecurrencePatternEvaluator.Evaluate or CalendarEvent.GetOccurrences and
• UNTIL must be specified as UTC (in most cases) according to RFC5545, but is treated as it would have the same time zone as DTSTART.

The following two test cases reproduce the problem (Europe/Vienna has offset UTC+2 in June 2018):

        [Test, Category("Recurrence")]
        public void UntilTimezoneTest()
        {
            var icalText = @"BEGIN:VCALENDAR
BEGIN:VEVENT
UID:ignore
DTSTAMP:20180613T154237Z
DTSTART;TZID=Europe/Vienna:20180612T180000
DTEND;TZID=Europe/Vienna:20180612T181000
RRULE:FREQ=HOURLY;UNTIL=20180612T170000Z
END:VEVENT
END:VCALENDAR
";

            var cal = Calendar.Load(icalText);
            var evt = cal.Events.First();
            var occurrences = evt.GetOccurrences(evt.DtStart);

            var occurrencesStartTimes = occurrences.Select(x => x.Period.StartTime).ToList();
            var expectedStartTimes = new[]
            {
                new CalDateTime(2018, 06, 12, 18, 0, 0, "Europe/Vienna"),
                new CalDateTime(2018, 06, 12, 19, 0, 0, "Europe/Vienna"),
            };

            Assert.IsTrue(expectedStartTimes.SequenceEqual(occurrencesStartTimes));
        }

        [Test, Category("Recurrence")]
        public void DTStartTimezoneTest()
        {
            var icalText = @"BEGIN:VCALENDAR
BEGIN:VEVENT
UID:ignore
DTSTAMP:20180613T154237Z
DTSTART;TZID=Europe/Vienna:20180612T180000
DTEND;TZID=Europe/Vienna:20180612T181000
RRULE:FREQ=HOURLY
END:VEVENT
END:VCALENDAR
";

            var cal = Calendar.Load(icalText);
            var evt = cal.Events.First();
            var ev = new EventEvaluator(evt);
            var startUtc = evt.DtStart.AsUtc;
            var occurrences = ev.Evaluate(evt.DtStart, startUtc, startUtc.AddSeconds(3601), false);
            var occurrencesStartTimes = occurrences.Select(x => x.StartTime).ToList();

            var expectedStartTimes = new[]
            {
                new CalDateTime(2018, 06, 12, 18, 0, 0, "Europe/Vienna"),
                new CalDateTime(2018, 06, 12, 19, 0, 0, "Europe/Vienna"),
            };

            Assert.IsTrue(expectedStartTimes.SequenceEqual(occurrencesStartTimes));
        }

(The problem was observed on a device running in the Europe/Vienna timezone. Not sure, whether this has any relevance to the outcome.)

[Edit]: Might be related to #197

[minichma]

@axunonb Problem still exists.

[axunonb]

See discussion on PR #574

[minichma]

Not sure I understand the connection to #574. I think this issue a different one and we should reopen it. Probably related to #197, but that's to be shown.

[axunonb]

Presently, it's the DTStartTimezoneTest() mentioned that still fails.

[axunonb]

If we really wanted to stick to DateTime? as arguments, the fix to make DTStartTimezoneTest()work was this change in EventEvaluator:

public override IEnumerable<Period> Evaluate(IDateTime referenceTime, DateTime? periodStart, DateTime? periodEnd, bool includeReferenceDateInResults)
{
    if (periodStart is { Kind: DateTimeKind.Utc })
        periodStart = new CalDateTime(periodStart.Value, "UTC").ToTimeZone(referenceTime.TzId).Value;

    if (periodEnd is { Kind: DateTimeKind.Utc })
        periodEnd = new CalDateTime(periodEnd.Value, "UTC").ToTimeZone(referenceTime.TzId).Value;
    ... oterh code

Looks like at some time there was awareness of the problem with DateTime arguments. Evaluator contains this method, that is never used with the current code base:

protected IDateTime ConvertToIDateTime(DateTime dt, IDateTime referenceDate)
{
    IDateTime newDt = new CalDateTime(dt, referenceDate.TzId);
    newDt.AssociateWith(referenceDate);
    return newDt;
}

UntilTimezoneTest() already works.