RecurrencePattern.Until DateTime property should be a CalDateTime

[rmtexas]

iCalendar spec says that the UNTIL format should match the same format as DTSTART. And if DTSTART specifies a TZID then the UNTIL should as well.
Because UNTIL is just a DateTime, there is no way to specify the timezone that should be applied the property.
So serialization of a recurrence comes out incorrectly.
Example:
BEGIN:VEVENT
DTEND;TZID=Central Standard Time:20190208T133000
DTSTAMP:20240702T173339Z
DTSTART;TZID=Central Standard Time:20190208T130000
RRULE:FREQ=DAILY;UNTIL=20190215T130000;WKST=SU
SEQUENCE:0
SUMMARY:Intro 1 : Recurring Appointment
UID:1
END:VEVENT

Notice that UNTIL has no TZID identifier applied. This is against spec.

[RemcoBlok]

Similarly the EXDATE also does not serialize a TZID. See #614

[RemcoBlok]

The way I read the standard, UNTIL must have the same value type as DTSTART. If DTSTART is a DATE, then UNTIL must be a DATE. If DTSTART is a DATE-TIME, then UNTIL must be a DATE-TIME. Oddly, when DTSTART specifies a TZID, UNTIL must be in UTC, and not in the TZID of DTSTART. See https://icalendar.org/iCalendar-RFC-5545/3-3-10-recurrence-rule.html.

"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"

It does not say the same thing about EXDATE having to be in UTC by the way, which is nice and consistent. See https://icalendar.org/iCalendar-RFC-5545/3-8-5-1-exception-date-times.html. For EXDATE you must be able to specify a TZID.

[minichma]

I agree that Until should not be specified as DateTime, as it is rather ambiguous (see #638). However, I fully agree with @RemcoBlok that according to the RFC, UNTIL MUST be specified either in UTC or as floating time (without TZ).

[axunonb]

Agree - now what does it mean in code? Only use UTC without implementing a floating time representation?
For CalDateTime in #638 a date/time with TZ null finally resolves to system's local time. Should TZ null be a floating time rather?

ical.net/Ical.Net/DataTypes/RecurrencePattern.cs

Lines 30 to 42 in e76da82

	public DateTime Until
	{
	get => _until;
	set
	{
	if (_until == value && _until.Kind == value.Kind)
	{
	return;
	}
	_until = value;
	}
	}

[minichma]

Should TZ null be a floating time rather?

Yes, I think this would be great!