new trimmer feature System.TimeZoneInfo.Invariant

[pavelsavara]

Motivation: when TryGetLocalTzFile is included it brings dependency to System.IO.File and its dependencies.

• new trimmer feature System.TimeZoneInfo.Invariant triggered by existing <InvariantTimezone>true</InvariantTimezone>
  • breaking change: it will prevent loading TZ info files from FS
• new System.TimeZoneInfo.Invariant getter also driven by DOTNET_SYSTEM_TIMEZONE_INVARIANT env variable
• cleanup ILLink.Substitutions.LegacyJsInterop.xml

Together with dotnet/sdk#45792

<InvariantTimezone>true</InvariantTimezone> is pre-existing feature.

After discussion below the implementation was unified to other platforms.
Explanation of the feature in comment below.

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[tarekgh]

Will it be acceptable to have DateTime.Now report wrong time when the invariant mode is enabled?

[pavelsavara]

Will it be acceptable to have DateTime.Now report wrong time when the invariant mode is enabled?

It will report UTC. Also this behavior already existed before this PR, here I'm just removing the ability to load TZ data from file system, when InvariantTimezone is true.

[tarekgh]

It will report UTC. Also this behavior already existed before this PR, here I'm just removing the ability to load TZ data from file system, when InvariantTimezone is true.

Thanks. I am not sure my question is answered :-) I am not questioning the PR more than questioning behavior. I guess users haven't run into this yet to report it. Returning UTC time when having TZ settings different than UTC is questionable. Maybe it is acceptable to the browser (especially servers) but wanted to know how we validated this assumption.
Maybe the explicit opt-in switch introduced here is better as users will explicitly enable and understand the consequences.

[pavelsavara]

Returning UTC time when having TZ settings different than UTC is questionable.

This is not how it works. You would not be able to change to non-UTC timezone. Let me show you:

given code

Console.WriteLine($"TimeZoneInfo.Local is {TimeZoneInfo.Local}");
Console.WriteLine($"DateTime.UtcNow is {DateTime.UtcNow}");
Console.WriteLine($"DateTime.Now is {DateTime.Now}");
var start = DateTime.UtcNow;
var timezonesCount = TimeZoneInfo.GetSystemTimeZones().Count;
await Delay(100);
var end = DateTime.UtcNow;
Console.WriteLine($"Found {timezonesCount} timezones in the TZ database in {end - start}");

TimeZoneInfo utc = TimeZoneInfo.FindSystemTimeZoneById("UTC");
Console.WriteLine($"{utc.DisplayName} BaseUtcOffset is {utc.BaseUtcOffset}");

try
{
    TimeZoneInfo tst = TimeZoneInfo.FindSystemTimeZoneById("Asia/Tokyo");
    Console.WriteLine($"{tst.DisplayName} BaseUtcOffset is {tst.BaseUtcOffset}");
}
catch (TimeZoneNotFoundException tznfe)
{
    Console.WriteLine($"Could not find Asia/Tokyo: {tznfe.Message}");
}

At 12:19 in Prague, you will get following output

TimeZoneInfo.Local is (UTC) UTC
DateTime.UtcNow is 1/9/2025 11:19:52 AM
DateTime.Now is 1/9/2025 11:19:52 AM
Found 0 timezones in the TZ database in 00:00:00.1200000
(UTC) UTC BaseUtcOffset is 00:00:00
Could not find Asia/Tokyo: The time zone ID 'Asia/Tokyo' was not found on the local computer.

[pavelsavara]

The <InvariantTimezone>true</InvariantTimezone> feature is opt-in and it means: "I don't care for timezones, please make the application as small as possible".

On the browser "operating system" we don't have TZ database pre-installed and so we are bundling it with each application. It's 300KB of download and some CPU time before the runtime can start. That's a lot for a web app, especially if your business logic is not about dates/times/calendars.

Before this PR the <InvariantTimezone>true</InvariantTimezone> would just stop shipping the data. But you would be able to ship your own and store it into emscripten virtual file system yourself. There are probably nobody in the world that knew this was possible.

I feel quite safe to make this change for the browser target.

@steveisok is this how it works for iOS too ?

Can I assume people are not shipping custom TZDIR there at the same time when they set <InvariantTimezone>true</InvariantTimezone> ?

[steveisok]

@steveisok is this how it works for iOS too ?

We don't ship anything custom on iOS or Android as we rely on whatever they do.

[pavelsavara]

@sbomer @MichalStrehovsky @tarekgh do you have further questions or feedback ?

[tarekgh]

Thanks @pavelsavara
I added a few minor comments. LGTM otherwise

[sbomer]

LGTM, thank you!

[pavelsavara]

/ba-g CI timeouts