GH-49952: [C++][Gandiva] Use timegm in date_time_test utilities

[Reranko05]

Rationale for this change

As discussed in PR #49887, the Gandiva datetime test utilities currently use mktime(), which interprets timestamps as local time and can behave differently across platforms and DST boundaries.

This PR switches the test utilities to timegm() so timestamps are interpreted consistently as UTC. This also removes the existing workaround logic in Epoch() that was previously needed for mktime() behavior on Windows/MSVC.

What changes are included in this PR?

• replaced mktime() with timegm() in date_time_test.cc
• added a small Windows compatibility mapping for _mkgmtime
• removed the existing DST/localtime workaround logic in Epoch()

Are these changes tested?

• Re-ran DateTimeTestProjector.TestFromUtcTimestamp repeatedly locally
• Re-ran the full gandiva-projector-test suite successfully

Are there any user-facing changes?

No.

• GitHub Issue: [C++][Gandiva] Use timegm in date_time_test utilities to avoid DST-dependent behavior #49952

[github-actions]

⚠️ GitHub issue #49952 has been automatically assigned in GitHub to PR creator.

[Reranko05]

@lriggs I tried the timegm() approach suggested in #49887 locally and opened this follow-up PR based on it.

[lriggs]

Thanks for the follow up.

[Reranko05]

@kou This PR has CI passing and an approval from @lriggs. Would you have time for a committer review when convenient? Thanks!

[Copilot]

Pull request overview

This PR updates Gandiva’s C++ datetime test utilities to interpret tm inputs as UTC (rather than local time) to avoid DST / timezone-dependent behavior in tests.

Changes:

• Replaced mktime() with timegm() in UTC timestamp utilities used by tests.
• Added a Windows compatibility mapping so timegm() resolves to _mkgmtime on _WIN32.
• Simplified Epoch() by removing the prior MSVC/localtime workaround.

Comments suppressed due to low confidence (1)

cpp/src/gandiva/tests/date_time_test.cc:95

• DaysSince() still uses mktime() (local time) even though Epoch()/MillisSince() now use UTC (timegm()). This keeps part of the date/time test utilities timezone/DST-dependent and contradicts the PR goal of making the tests deterministic. Switch DaysSince() to timegm() (with the existing Windows mapping) and update the error message accordingly.

  // time_t is an arithmetic type on both POSIX and Windows, we can simply
  // subtract to get a duration in seconds.
  return static_cast<int64_t>(ts - base_line) * 1000 + millis;
}

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[kou]

+1

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit d0c59fa.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 9 possible false positives for unstable benchmarks that are known to sometimes produce them.