Midnight Transitions in NodaTime 2.x vs 3.x

Question

I'm trying to reproduce a different behavior of NodaTime 2.x versus 3.x. As described here:

This could be considered a bugfix, but it is an incompatible change nonetheless. Transitions that would be represented as 24:00 in the IANA time zone database ("following day midnight") are instead represented as 23:59:59.999 in the Windows database. BclDateTimeZone in Noda Time 1.x and 2.x will use this value as provided, resulting in (for example) the start of a day being reported as one millisecond earlier than intended (but matching the behaviour of TimeZoneInfo). Noda Time 3.0 will instead recognise this pattern and interpret it as midnight of the following day, matching the 'correct' behaviour.

Here is what I tried so far:

TimeZoneInfo tzi = TimeZoneInfo.Utc;
BclDateTimeZone bclDt = BclDateTimeZone.FromTimeZoneInfo(tzi);

Instant i1 = Instant.FromDateTimeUtc(new DateTime(2020, 9, 10, 22, 59, 59, 999, DateTimeKind.Utc));
Instant i2 = Instant.FromDateTimeUtc(new DateTime(2020, 9, 10, 23, 00, 00, 000, DateTimeKind.Utc));
ZonedDateTime zdt1 = new ZonedDateTime(i1, bclDt);
ZonedDateTime zdt2 = new ZonedDateTime(i2, bclDt);

Console.WriteLine(zdt1.PlusHours(1));
Console.WriteLine(zdt2.PlusHours(1));
Console.ReadKey();

The output (Framework 4.8, NodaTime 3.x), the same as for 2.x:

2020-09-10T23:59:59 UTC (+00)
2020-09-11T00:00:00 UTC (+00)

From my understanding in NodaTime 3.x both dates should output September 11. In contrast, in NodaTime 2.x the first one should be as it is in the output (September 10). But something isn't quite right here. What am I missing?

Answer 1

Jon Skeet's comment and the linked GitHub issue explain why it doesn't work and how to reproduce it. But here is a working example just in case:

DateTimeZone bclDt = DateTimeZoneProviders.Bcl["E. South America Standard Time"];

// this equals to "AtStartOfDay" of LocalDate at the same DateTimeZone
Instant i1 = Instant.FromDateTimeUtc(new DateTime(2012, 10, 21, 02, 59, 59, 999,  DateTimeKind.Utc));
Instant i2 = Instant.FromDateTimeUtc(new DateTime(2012, 10, 21, 03, 59, 59, 999, DateTimeKind.Utc));
ZonedDateTime zdt1 = new ZonedDateTime(i1, bclDt);
ZonedDateTime zdt2 = new ZonedDateTime(i2, bclDt);

Console.WriteLine(zdt1.PlusHours(-1));
Console.WriteLine(zdt2.PlusHours(-1));

Console.ReadKey();

The output for NodaTime 2.x:

2012-10-20T22:59:59 E. South America Standard Time (-03)
2012-10-21T00:59:59 E. South America Standard Time (-02)

The output for NodaTime 3.x:

2012-10-20T22:59:59 E. South America Standard Time (-03)
2012-10-20T23:59:59 E. South America Standard Time (-03)

In case someone wandering, how have I got the "magic" initial value of DateTime(2012, 10, 21, 02, 59, 59, 999) - by modifying Jon Skeet's example, posted on the linked GitHub issue:

DateTimeZone bclDt = DateTimeZoneProviders.Bcl["E. South America Standard Time"];

LocalDate date = new LocalDate(2012, 10, 21);
ZonedDateTime zoned = bclDt.AtStartOfDay(date);
Instant inst = zoned.ToInstant();
ZonedDateTime utc = inst.InUtc();
Console.WriteLine(ZonedDateTimePattern.ExtendedFormatOnlyIso.Format(utc));
Console.ReadKey();

The output is for NodeTime 2.x:

2012-10-21T02:59:59.999 UTC (+00)

Thanks, Jon!