[browser] Fix DateTime marshalling to be timezone-independent

[pavelsavara]

Problem

ToJS(DateTime) used new DateTimeOffset(value) which applies the local timezone offset when DateTime.Kind is Unspecified or Local. The ToManaged side always returns .UtcDateTime (offset=0). This asymmetry causes the roundtripped DateTime to shift by the local UTC offset.

With Emscripten 3.1.56, the WASM environment effectively ignored the TZ environment variable, so local time always equaled UTC — masking the bug. Emscripten 5.0.6 correctly processes TZ, exposing the issue on any machine with a non-UTC timezone.

Failing tests (CET/CEST, +02:00)

• DateTimeMaxValueBoundaryCondition — 1h shift (CET for Dec 31)
• DateTimeMarshallingLosesMicrosecondComponentPrecisionLoss — 2h shift (CEST for Apr 1)
• DateTimeMinValueBoundaryCondition — ArgumentOutOfRangeException (MinValue + offset underflows year 0)
• JsExportFuncOfDateTime_Argument_OverflowNETDateTime — MaxValue+60s no longer overflows
• JsExportFunctionDateTimeDateTime — DateTime.Now roundtrip loses offset hours

Fix

Replace new DateTimeOffset(value) with new DateTimeOffset(value.Ticks, TimeSpan.Zero) in both ToJS(DateTime) and ToJS(DateTime?). This reinterprets ticks as UTC, matching the ToManaged side which already returns .UtcDateTime.

The DateTimeOffset overloads are unaffected — ToUnixTimeMilliseconds() already accounts for the stored offset.

Before (broken in non-UTC timezone)

C# ToJS:    new DateTimeOffset(20:00 local, +02:00) → epoch for 18:00 UTC
JS:         new Date(epoch) → 18:00 UTC internally
C# ToManaged: FromUnixTimeMilliseconds(...).UtcDateTime → 18:00 (clock value shifted!)

After (timezone-independent)

C# ToJS:    new DateTimeOffset(20:00 ticks, +00:00) → epoch for 20:00 UTC
JS:         new Date(epoch) → 20:00 UTC internally
C# ToManaged: FromUnixTimeMilliseconds(...).UtcDateTime → 20:00 (clock value preserved)

Test coverage

Added TZ=Europe/Berlin to WasmXHarnessMonoArgs in the test csproj so CI (which runs in UTC) exercises the same non-UTC code paths as local development.

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[Copilot]

Pull request overview

This PR fixes DateTime marshalling between .NET and JavaScript in browser/WASM so roundtrips are timezone-independent, avoiding tick shifts when the local timezone is non-UTC (notably exposed with newer Emscripten behavior around TZ).

Changes:

• Marshal DateTime/DateTime? to JS using a zero-offset DateTimeOffset(value.Ticks, TimeSpan.Zero) to avoid applying the local timezone offset.
• Configure the WASM unit test run to use a non-UTC timezone (TZ=Europe/Berlin) so CI exercises the previously-masked code paths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Marshaling/JSMarshalerArgument.DateTime.cs	Makes DateTime and nullable DateTime marshalling interpret ticks as UTC (offset 0) when converting to Unix epoch milliseconds.
src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System.Runtime.InteropServices.JavaScript.Tests.csproj	Sets TZ=Europe/Berlin for WASM test execution to validate behavior in a non-UTC timezone in CI.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[maraf]

Looks good to me and my copilot 👍

[pavelsavara]

/ba-g known CI badexit