date: implement E and O locale modifiers

[naoNao89]

This adds support for POSIX locale extension modifiers:

• E modifiers (%EY, %Ey, %EC, %EB) for alternative representations
• O modifiers (%Od, %Om, etc.) falls back to standard numerals

Implemented via ICU integration for calendar conversions and localized month names.

Note: Full alternative numeric system support (Arabic-Indic digits, Devanagari, blabla...) is deferred to a future enhancement as it requires ICU FixedDecimalFormatter integration

ref:

• POSIX strftime(): E/O conversion semantics and fallback behavior.
• GNU coreutils: Padding and other flags: modifier placement, flags, and field widths.
• GNU libc: Calendar time formatting: GNU-specific locale formatting behavior.

Fixes #10958

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/date-locale-hour. tests/date/date-locale-hour is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[sylvestre]

hi, this one can't be merged as it is: the build or the tests fail; it needs a rebase
could you please have a look? otherwise, i will close it, thanks

[naoNao89]

oke

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[codspeed]

Merging this PR will degrade performance by 6.91%

⚠️ 123 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• cksum_crc
• cksum_default
• cksum_raw_output
• dd_copy_with_seek
• dd_copy_with_skip
• single_date_now
• dd_copy_8k_blocks
• df_deep_directory
• dd_copy_4k_blocks
• dd_copy_default
• …and 113 more

ℹ️ 1 benchmark has no flame graph

The flamegraph could not be generated because too much time was spent in runtime internals, making the visualization not useful for performance analysis.

• sort_dictionary_order[500000]

❌ 2 regressed benchmarks
✅ 399 untouched benchmarks
⏩ 70 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	file_tz_abbreviations	18.9 ms	20.3 ms	-6.93%
❌	Simulation	file_iso_dates	18.9 ms	20.4 ms	-6.89%

Tip

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

---

_{Comparing naoNao89:date-eo-modifiers (c5c9326) with main (6ef5fe8)²}

Footnotes

1. 70 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (df66079) during the generation of this report, so 6ef5fe8 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/follow-symlink is no longer failing!