Fix TimeZoneInfo.ConvertTime producing wrong results near DateTime.MinValue/MaxValue

[tarekgh]

Fix

TimeZoneInfo.ConvertTime returns incorrect results when converting a DateTime near DateTime.MinValue between positive-offset time zones (or near DateTime.MaxValue between negative-offset time zones).

Root Cause

ConvertTime works by first converting the input to UTC, then from UTC to the destination time zone. When the intermediate UTC value has negative ticks (outside DateTime range), SafeCreateDateTimeFromTicks clamps it to DateTime.MinValue (ticks = 0). The destination offset is then added to this clamped value, producing a wrong result.

Example: Converting 0001-01-01 01:13:15 from UTC+4 to UTC+3:

• Intermediate UTC ticks = 1h13m15s - 4h = negative (before epoch)
• Clamped to DateTime.MinValue (ticks = 0)
• Add destination offset (+3h) → 0001-01-01 03:00:00 ❌
• Expected: 0001-01-01 00:13:15 ✅

Fix

Changed TryLocalToUtc to return raw long utcTicks instead of a clamped DateTime. ConvertTime now computes the final result from raw ticks (utcTicks + destOffset.Ticks), only creating a clamped DateTime for transition-table offset lookups. This avoids precision loss from double-clamping when the intermediate UTC value is outside DateTime range but the final result is valid.

Testing

• Added 7 new [Theory] test cases using custom time zones (cross-platform):
  • 4 cases for near-DateTime.MinValue with positive offsets (including exact repro from issue)
  • 3 cases for near-DateTime.MaxValue with negative offsets (symmetric)
• All 68,715+ System.Runtime.Tests pass with zero failures.
• Local benchmarking shows no performance regression.

Fixes #126940

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[Copilot]

Pull request overview

Fixes TimeZoneInfo.ConvertTime incorrect results when converting DateTime values near DateTime.MinValue/DateTime.MaxValue by avoiding clamping of the intermediate UTC value before applying the destination offset.

Changes:

• Refactored TryLocalToUtc to return raw utcTicks (long) instead of a clamped DateTime.
• Updated ConvertTime to compute final local ticks from utcTicks + destOffset.Ticks, using a clamped UTC DateTime only for offset lookup.
• Added new cross-platform regression tests using custom fixed-offset time zones for Min/Max boundary scenarios.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.cs	Computes conversion result using raw UTC ticks to avoid incorrect double-clamping near DateTime bounds.
src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.Cache.cs	Changes TryLocalToUtc to output raw UTC ticks (can be outside DateTime range).
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/TimeZoneInfoTests.cs	Adds regression coverage for DateTime conversions near Min/Max values using custom offset-only zones.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.