date input: bare timezone abbreviations (`ut`, `UT`, `gmt`, …) rejected

[sylvestre]

Summary

GNU date -d accepts bare timezone abbreviations (UT, ut, GMT, gmt, …) and interprets them as "now, expressed in that timezone". parse_datetime rejects them as invalid.

Found by a fuzz_date run on uutils/coreutils.

Reproduction

$ TZ=UTC+1 /usr/bin/date -d 'ut' '+%Z'
UTC

$ TZ=UTC+1 target/debug/coreutils date -d 'ut' '+%Z'
date: invalid date 'ut'
$ echo $?
1

$ TZ=UTC+1 /usr/bin/date -d 'GMT' '+%Z'
UTC

$ TZ=UTC+1 target/debug/coreutils date -d 'GMT' '+%Z'
date: invalid date 'GMT'

Test (in uutils/coreutils integration suite)

#[test]
fn test_date_bare_timezone_abbreviation() {
    for input in ["ut", "UT", "gmt", "GMT"] {
        new_ucmd!()
            .env("TZ", "UTC+1")
            .arg("-d")
            .arg(input)
            .arg("+%Z")
            .succeeds()
            .stdout_is("UTC\n");
    }
}