fix(offset): accept MEZ, MESZ, MEST and KST abbreviations

[ARMeeru]

timezone_name_to_offset documents its own scope as matching GNU:

GNU date only supports a subset of these. We support the same subset as GNU date.

Four abbreviations GNU accepts are missing from the table, so they fail here:

$ TZ=UTC date -u -d '2024-01-15 12:00 MEZ' +%H:%M
11:00

MEZ is +1, MESZ and MEST are +2, and KST is +9.

How these four were picked

Rather than adding only the ones I happened to run into, I compared the crate against GNU across 100 candidate abbreviations: everything already in this table, plus the ones uutils/coreutils carries in date.rs, plus a spread of common world abbreviations. GNU accepted 72 of them and the crate agreed on 63. The other nine fall into two groups.

Four the crate rejects outright. Those are in this PR.

Five where both accept but the offsets disagree: ADT, AST, BST, GST and SST. Every one of those is a genuinely ambiguous name (Atlantic or Arabia, British or Bangladesh, Gulf or Guam), and changing a value that already exists is a different sort of decision from filling a hole. I left them alone. Happy to open an issue with the measurements if you want them tracked.

Where the offsets came from

Each one was measured by running GNU date and comparing instants, for example:

$ TZ=UTC date -u -d '2024-01-15 12:00 KST' +%H:%M
03:00

GNU coreutils 9.1. These came from running GNU date and reading its output, not from reading its source.

Tests

The four are added to the existing timezone_name_without_offset list. That test fails before the change and passes after. The rest of the suite is unaffected.

Context

This gap is part of why uutils/coreutils keeps a separate abbreviation table in date.rs, which came up in uutils/coreutils#13865. This change does not remove that table on its own, and the two are independent.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.33%. Comparing base (46fb737) to head (5266ff2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #321   +/-   ##
=======================================
  Coverage   99.33%   99.33%           
=======================================
  Files          21       21           
  Lines        4051     4061   +10     
  Branches      129      129           
=======================================
+ Hits         4024     4034   +10     
  Misses         26       26           
  Partials        1        1           

Flag	Coverage Δ
macos_latest	99.33% <100.00%> (+<0.01%)	⬆️
ubuntu_latest	99.33% <100.00%> (+<0.01%)	⬆️
windows_latest	13.83% <0.00%> (-0.04%)	⬇️

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[codspeed-hq]

Merging this PR will degrade performance by 3.66%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 20 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	parse_weekday	62.1 µs	64.5 µs	-3.66%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ARMeeru:fix/missing-gnu-timezone-abbreviations (5266ff2) with main (46fb737)}