Compare DateTime instances respecting their Kind in Test-Item (#25013)

[FstTesla]

PR Summary

Fix #25013.

When comparing DateTime instances provided as parameter -OlderThan/-NewerThan in cmdlet Test-Item, now the Kind (local, UTC or unspecified) is correctly taken into account.

Tests have also been updated, but with a caveat! ⚠️
The impacted tests acquire significance when run in a timezone with some test-dependent criteria. To ensure the test behavior behavior in a cross-platform, revertable, scoped (in-process, non-system-wide) fashion, I temporarily set a custom timezone for the duration of each test. Unfortunately, the in-process local timezone can be set only via reflection, so any improvements to this tweaky approach are welcome.

PR Context

PR Checklist

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• Breaking changes
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• Testing - New and feature
  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed

[Copilot]

Pull Request Overview

This PR fixes a bug in the Test-Path cmdlet where DateTime comparisons with -OlderThan and -NewerThan parameters were not correctly respecting the DateTime Kind property (Local, UTC, or Unspecified). The fix ensures that all DateTime values are properly converted to UTC before comparison.

Key Changes:

• Updated the comparison logic to convert DateTime instances to UTC while respecting their Kind property
• Modified test setup to use UTC-based DateTime values and file timestamps
• Added timezone manipulation in tests using reflection to ensure cross-platform test reliability

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
FileSystemProvider.cs	Fixed DateTime comparison logic to convert to UTC while handling Unspecified DateTimeKind
Test-Path.Tests.ps1	Updated tests to use UTC DateTime values and added timezone manipulation for reliable cross-platform testing

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[Copilot]

When DateTimeKind is Unspecified, using DateTime.SpecifyKind to treat it as UTC could lead to incorrect comparisons. An Unspecified DateTime should typically be assumed to be local time and converted using DateTime.ToUniversalTime(), or the behavior should be explicitly documented. The current implementation may produce unexpected results when users pass DateTime values without specifying a Kind.

[iSazonov]

I agree with Copilot:

Suggested change

	DateTime olderThanUtc = olderThan.Kind == DateTimeKind.Unspecified
	? DateTime.SpecifyKind(olderThan, DateTimeKind.Utc)
	: olderThan.ToUniversalTime();
	DateTime olderThanUtc = olderThan.ToUniversalTime();

[FstTesla]

Initially I thought that interpreting DateTimeKind.Unspecified as UTC would be more transparent; however, also in order to stick to the previous behavior, now I agree with the suggestion.
I will apply the same also below.

[Copilot]

When DateTimeKind is Unspecified, using DateTime.SpecifyKind to treat it as UTC could lead to incorrect comparisons. An Unspecified DateTime should typically be assumed to be local time and converted using DateTime.ToUniversalTime(), or the behavior should be explicitly documented. The current implementation may produce unexpected results when users pass DateTime values without specifying a Kind.

[FstTesla]

@microsoft-github-policy-service agree