Wrong TimeZone Information in NET Core 3.1 and NET 5

[macriluis]

Describe the bug

Using a Windows PC, you set your Time Zone as (UTC-03:00) City of Buenos Aires
In Blazor WebAssembly client, the Time Zone is UTC.

To Reproduce

Create a new Blazor WebAssembly app.

In Index.razor add:

Your Time: @DateTime.Now.ToString()
Your Time Zone: @TimeZoneInfo.Local.DisplayName

In Control Panel or Configuration, set the Time Zone as (UTC-03:00) City of Buenos Aires
In Blazor WebAssembly client, the Time zone is UTC.
Wrong.

Further technical details

• ASP.NET Core version
• Include the output of dotnet --info

SDK de .NET Core (reflejando cualquier global.json):
Version: 5.0.100-rc.2.20479.15
Commit: da7dfa8840

Entorno de tiempo de ejecución:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.100-rc.2.20479.15\

Host (useful for support):
Version: 5.0.0-rc.2.20475.5
Commit: c5a3f49

.NET SDKs installed:
3.1.300-preview-015115 [C:\Program Files\dotnet\sdk]
3.1.400-preview-015151 [C:\Program Files\dotnet\sdk]
3.1.403 [C:\Program Files\dotnet\sdk]
5.0.100-rc.2.20479.15 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.0-rc.2.20475.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.0-rc.2.20475.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.0-rc.2.20475.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

• The IDE (VS / VS Code/ VS4Mac) you're running on, and it's version
  Visual Studio 2019 16.8 Preview 4

[Dotnet-GitSync-Bot]

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[pranavkm]

@lewing \ @kjpou1 looks like something's amiss with the TZ database. The TZ value is America/Buenos_Aires for the location. On my Ubuntu machine, a /usr/share/zoneinfo/America/Buenos_Aires but not in the TZ file carried by .NET.

[lewing]

/cc @tqiu8

[ghost]

Tagging subscribers to this area: @CoffeeFlux
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[macriluis]

@lewing Is there a workaround for this?
The only way I have found, is to tell the users to change the TimeZone in their computers.
But, this is not working good, some users forget to do it, others simply does not want to do that change.
I need to use the TimeZone (-3) for Argentina.
Thanks in advance.

[lewing]

@macriluis what does Intl.DateTimeFormat().resolvedOptions().timeZone print from the javascript console on those machines?

[pranavkm]

@lewing on my machine it prints America/Buenos_Aires

[macriluis]

@lewing on my machine, Windows 10, using Edge or Chrome the javascript console says America/Buenos_Aires

[tqiu8]

So the timezone id given by the browser (Intl.DateTimeFormat().resolvedOptions().timeZone) is different from the id used by the tz data (e.g. America/Buenos_Aires in browser, vs America/Argentina/Buenos_Aires in tzdb). This problem extends to any other timezone that has three levels instead of two.

[lewing]

@macriluis the issue appears to be solvable by replacing the dotnet.timezones.blat file from the runtime pack with the eventual result of #43786

We'll backport the fix once everything is verified.

[macriluis]

@lewing thank you very much!