date: support MMDDhhmm[[CC]YY][.ss] positional argument for setting date

[aguimaraes]

Fixes #6398

The date command accepts a bare positional argument to set the system clock:

date 03271430 # March 27, 14:30, current year
date 0327143025 # 2-digit year (25 -> 2025)
date 032714302025 # 4-digit year
date 032714302025.30 # with seconds

Previously, any positional argument without a + prefix was rejected with "invalid date". This was already documented in the usage string but never implemented.

The parser lives in date.rs rather than the parse_datetime crate because this format is specific to the date command positional argument. It follows the existing pattern in this codebase where touch has its own parse_timestamp() for the similar [[CC]YY]MMDDhhmm[.ss] format.

The implementation rearranges the input into CCYYMMDDhhmm[.ss] and delegates to jiff::fmt::strtime::parse for validation, same approach as touch::parse_timestamp(). Two-digit years use the same century rule as touch: 00-68 -> 20xx, 69-99 -> 19xx.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/rm/isatty (fails in this run but passes 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/tee/tee is no longer failing!
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/cp/link-heap is now passing!

[codspeed-hq]

Merging this PR will improve performance by 4.97%

⚠️ 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 improved benchmark
✅ 301 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	cp_large_file[16]	278.1 µs	265 µs	+4.97%

---

_{Comparing aguimaraes:date-positional-set-format (5e0e12b) with main (15f57d0)}

Footnotes

1. 46 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. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!