date: add military time zone support

[jfinkels]

In GNU date version 8.32,

$ date --version | head -n 1
date (GNU coreutils) 8.32.199-88562
$ date -u -d "05:00C" +%T
02:00:00

The time zones labeled "A", "B", "C", etc. are UTC+1, UTC+2, UTC+3, etc. and these are called military time zones in RFC 5322. (In earlier versions of GNU date, it appears there was a bug in this behavior, so using version 8.32 or later is important: https://lists.gnu.org/r/bug-gnulib/2019-08/msg00005.html )

But uutils doesn't support this:

$ ./target/debug/coreutils date -u -d "05:00C" +%T
date: invalid date '05:00C'

This is because it relies on the chrono crate for parsing, and chrono doesn't support military time zones. Here is the test case that demonstrates it: https://github.com/chronotope/chrono/blob/3467172c31188006147585f6ed3727629d642fed/src/format/parse.rs#L759 Here is the relevant section of RFC 5322, Section 4.3:

The 1 character military time zones were defined in a non-standard
way in [RFC822] and are therefore unpredictable in their meaning.
The original definitions of the military zones "A" through "I" are
equivalent to "+0100" through "+0900" respectively; "K", "L", and "M"
are equivalent to "+1000", "+1100", and "+1200" respectively; "N"
through "Y" are equivalent to "-0100" through "-1200" respectively;
and "Z" is equivalent to "+0000". However, because of the error in
[RFC822], they SHOULD all be considered equivalent to "-0000" unless
there is out-of-band information confirming their meaning.

[stale]

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