How to get correct date time by TimeZoneInfo when time zone offset is changed in history

[Li7ye]

Now, Singapore uses Singapore Time (utc + 8), but due to historical reasons, Singapore has revised the zone time several times. Ref : link.

var utc = DateTime.SpecifyKind(new DateTime(1942, 5, 1), DateTimeKind.Utc) ;
var timeZone = TimeZoneInfo.FindSystemTimeZoneById("Singapore Standard Time");
var sgDate = TimeZoneInfo.ConvertTimeFromUtc(utc, timeZone); // expected 1942-5-1 +09:00 , but output 1942-5-1 +08:00

How to handle this scenario in .NET?

[huoyaoyuan]

See #53519 (comment)

It depends on the time zone data source in use.

[Li7ye]

It means don't work on Windows？

[elgonzo]

If you need something now that works on Windows (and other platforms targetted by .NET), try using the NodaTime package which supports IANA TZDB (time zone database).

Your code example adopted for NodaTime could for example look like this:

var instantTime = Instant.FromUtc(1942, 5, 1, 0, 0);
var zone = DateTimeZoneProviders.Tzdb.GetZoneOrNull("Asia/Singapore");
var zonedTime = instantTime.InZone(zone);

Console.WriteLine(zonedTime);

with the output of this example being something like 1942-05-01T09:00:00 Asia/Singapore (+09).

[Clockwork-Muse]

Note that if you're doing a lot of work with timezones you probably want to be using NodaTime anyways, as the runtime's native support for timezone-related operations is limited.

[Li7ye]

If on Linux, can guarantee correctness by using TimeZoneInfo.ConvertTimeFromUtc?

[Clockwork-Muse]

That depends on what you mean by "correctness". The underlying requirement is that there be (accurate) historical data.

Strictly speaking, there isn't any guarantee that the data will be present on any operating system/machine, including Linux. In fact, if you're dealing with a containerized application, by default no timezone information (at all) is present/available.

That said, generally Linux has (more) historical data for more zones than Windows does, yes.
Keep in mind that this data is updated regularly, and can change over time.

[Li7ye]

@Clockwork-Muse I got it, thanks for your response.