fix(date): accept HH:MM date-times and zone suffixes in -d

[xmakro]

What changed

date -d now accepts ISO date-times without seconds and with a zone. touch -d and find -newermt share the parser and gain the same forms.

• YYYY-MM-DD HH:MM (with T or a space), and an optional fraction after the seconds
• a trailing UTC/GMT, Z, or a numeric offset (+0200, +02:00, -05), so git's %ci output parses

The change is a few more chrono formats next to the existing ones; chrono validates the fields and the offset range.

Why

Only YYYY-MM-DD and YYYY-MM-DD HH:MM:SS without a zone were accepted, so common inputs such as a minute-precision timestamp, a UTC log line or a git commit date failed with invalid date.

Before / After

$ date -u -d '2026-10-08 14:30' '+%F %T'
Before: date: invalid date '2026-10-08 14:30'
After:  2026-10-08 14:30:00

$ date -u -d '2026-10-08 14:30:15 UTC' '+%F %T'
Before: date: invalid date '2026-10-08 14:30:15 UTC'
After:  2026-10-08 14:30:15

$ date -u -d '2026-10-08 14:30:15 +0200' '+%F %T'
Before: date: invalid date '2026-10-08 14:30:15 +0200'
After:  2026-10-08 12:30:15

Every After line matches GNU date 9.12. Compared with GNU on 31 date-time inputs in UTC and America/Los_Angeles: the only differences are rare forms this still rejects (a comma fraction 14:30:15,5, an exact -2400 offset, 2026-10-08Z), and 12:30:60, which bashkit already accepted before this change.

Risk

• Low
• Inputs that failed before now parse; inputs that parsed before parse the same way.

Checklist

• Tests added or updated: unit tests, spec cases in date.test.sh, three rows in the GNU date differential test
• Backward compatibility considered