[iOS] TimeZoneInfo.FindSystemTimeZoneById throws TimeZoneNotFoundException on iOS but works on Windows and Android

[DulanyaPerera]

Description

When using TimeZoneInfo.FindSystemTimeZoneById in a .NET MAUI app, the method throws a TimeZoneNotFoundException on iOS for valid timezone IDs like "Eastern Standard Time" and "US Mountain Standard Time", even though the same code works correctly on Windows and Android.

Steps to Reproduce

Steps to reproduce

1. Create a new .NET MAUI app.
2. Add the following code in a shared C# context:
   var tz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
3. Run the app on an iOS device or simulator.

Expected Behavior

The method should return the correct 'TimeZoneInfo' object on all platforms, including iOS.

Actual Behavior

System.TimeZoneNotFoundException:

Link to public reproduction project repository

No response

Version with bug

10.0.0-preview.4

Is this a regression from previous behavior?

Not sure, did not test other versions

Last version that worked well

No response

Affected platforms

iOS

Affected platform versions

No response

Did you find any workaround?

No response

Relevant log output

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-datetime
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @ivanpovazan, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[deepika2509]

This issue has been verified in Visual Studio Code 1.99.3 with MAUI versions (10.0.0-preview.4.25263.4, 9.0.0 & 9.0.70. It can be reproduced on iOS platform. It working fine on the Android and Windows platforms.

[Clockwork-Muse]

for valid timezone IDs like "Eastern Standard Time" and "US Mountain Standard Time"

These are Windows Timezone ids, and have almost no use outside of that platform. I'm not sure why it's working on Android (if you're running in an emulator, maybe it's using the host zones?). It will fail on Linux systems.

Expected Behavior
The method should return the correct 'TimeZoneInfo' object on all platforms, including iOS.

TimeZone information is provided by the host platform, and you're not guaranteed that:

• The same zones are present on all platforms
• The same zone has the same rules on all platforms.
  • The same zone has the same rules on all instances of a platform (ie, different iPhones can have different rules, even though they're all iOS)

For compatibility reasons, you should be using the IANA/Olson timezone ids. On Windows, this means you may need to adjust your code to convert the ids.

That said, in most cases for end-user devices this is a non-issue, because the "current"/"system" timezone is an object that's just available to you without needing to specify an Id.