[kirre-bylund]

Implements T9a: the Windows-to-IANA timezone converter, plus the TimezoneProvider seam the owner asked for.

Why

The Game API takes an IANA timezone on a session start. A Windows game knows its timezone by a Windows name, so without a converter it either sends nothing and gets UTC server-side, or sends a name the API rejects. UTC puts local-day boundaries on the wrong day: daily logins, event windows and once_per_day buckets all land wrong.

The table is generated, not hand-written

• data/windows-to-iana.tsv — 137 pairs, the source of truth.
• tools/generate_timezone_table.py — produces src/core/timezone_table.inc.
• A new generated-files CI job fails if the two drift.

The data comes from the Unity SDK's LootLockerTimezoneConverter, which is the mature implementation, with one gap it has filled: Russian Standard Time appears only in its reverse table, so a Windows machine reporting it would fall through to UTC.

The platform clock is a seam

Timezone and locale are both potentially user-configurable in a game, and every engine exposes them differently, so the OS value is frequently not what the player chose. An SDK that only reads the OS is confidently wrong on a machine whose owner picked a different timezone in-game.

• TimezoneProvider — public interface, one method, injected through Config::timezone_provider.
• SystemTimezoneProvider — the default: GetDynamicTimeZoneInformation on Windows, TZ then /etc/localtime on POSIX, UTC on platforms we do not cover.
• Resolution order: Config::timezone, then the injected provider, then the system provider, then Etc/UTC.
• The default is documented as possibly wrong in the header, not presented as authoritative.

Verification

• 19 test functions: the fixture of known pairs, case-insensitivity, unknown and empty input, reverse conversion, the reversed IANA sign, clamping, every normalize form, the fallback, default casing, and the full resolution order through a FixedProvider.
• Zero warnings on MSVC 19.33; all 5 suites pass.
• The generated-files check was verified by hand-editing the table and watching it fail.
• The timezone tests were verified by breaking the reversed-sign convention and watching 6 checks fail.
• LOOTLOCKER_MODULE_ONLY=auth still builds, so the module boundary holds.

Not covered here

The task's verification also asks for "a session start from a Windows timezone name sends the IANA equivalent". That needs T14. ResolveTimezone is the function a session start will call, and it is covered directly.

Spec

lootlocker/index#1878 records the seam as a revision to the timezone section.

[Copilot]

🟡 Changes recommended

Active critical and moderate findings remain, including likely build failures on supported POSIX and unsupported non-Windows targets, unreliable numeric clamping, incomplete timezone recognition, and a contract conflict around implicit system detection.

6 open issues

• Include direct declarations for snprintf and POSIX readlink · New
• Guard POSIX detection and use UTC on unsupported platforms · New
• Clamp the parsed offset before converting it to int · New
• Avoid implicit environment and file reads in timezone resolution · New
• Validate IANA zones independently of the reduced mapping table · New
• Add the documented Pago Pago and Niue timezone mappings · New

What changed in this PR

Adds the Windows-to-IANA timezone conversion layer, generated mapping data, injectable timezone providers, platform fallback resolution, tests, and generated-file CI validation for future session-start integration.

Changes:

• Adds the 137-entry Windows/IANA source table, generator, and committed generated C++ table.
• Adds public conversion, normalization, provider, and configuration APIs.
• Adds platform detection, resolution precedence, timezone tests, CMake integration, and CI drift checking.

The review identified unresolved compatibility, portability, compilation, normalization, and system-detection issues. This pull request requires changes.

File	Summary	Review notes
tools/​generate_timezone_table.py	Generates the C++ timezone table from TSV source data.	—
tests/​test_timezone.cpp	Tests conversion, normalization, offsets, fallback, and provider resolution.	—
tests/​CMakeLists.txt	Registers the timezone test executable and CTest entry.	—
src/​core/​timezone.cpp	Implements conversion, normalization, provider resolution, and platform detection.	Critical: add required direct headers (1 vote); guard POSIX APIs on unsupported platforms (2 votes). Moderate: clamp numeric values before casting (2 votes); preserve canonical IANA casing (1 vote); validate broader IANA names (3 votes); address implicit environment/file reads (4 votes); support leading-colon TZ values (1 vote).
src/​core/​timezone_table.inc	Contains the generated timezone mapping table.	—
include/​lootlocker/​timezone.h	Defines the public timezone conversion and provider APIs.	Nit: add or document the Pacific/Pago_Pago and Pacific/Niue reverse mappings (2 votes).
include/​lootlocker/​config.h	Adds timezone override and provider configuration fields.	Moderate: preserve positional aggregate-initialization compatibility when adding timezone_provider (1 vote).
data/​windows-to-iana.tsv	Provides the source-of-truth Windows-to-IANA mappings.	—
CMakeLists.txt	Adds the timezone implementation to the core library.	—
.github/​workflows/​build.yml	Adds generated-file consistency validation to CI.	—

_{Review effort: Lite}