time: Parsing of GMT+XX offsets is broken

[jwebb]

What version of Go are you using (go version)?

1.19.1

Does this issue reproduce with the latest release?

Yes

What did you do?

https://go.dev/play/p/e5jcM1fIJm9?v=gotip (case 4 is the bug)

What did you expect to see?

GMT+2, if it is parsed at all, ought to be treated the same as +0200 (or -0200, according to Posix).

What did you see instead?

It causes the time to be parsed as UTC, but be labelled as though an offset were applied.

When applying a known zone abbreviation the time is shifted at https://cs.opensource.google/go/go/+/refs/tags/go1.19.2:src/time/format.go;l=1365;drc=85196fc982ead65ea56c377c2e381eabff329773;bpv=0;bpt=1 - it seems the same should be done at L1373.

However, this GMT+XX format parsing has a bunch of special-case code, supported by a single test, which checks the wrong input, and it's been this way forever without anyone noticing apparently. And as noted in #40472, it's undocumented and interacts confusingly with the ISO/RFC822 style. Further, as noted in my case 3 above, the direction of the offset in this notation has conflicting conventions. So I'd suggest just removing this from the parser entirely.

(I encountered this only because I was reading the code looking for a way to detect my case 5 and turn it into an error - which sadly seems impossible in general without separately parsing zoneinfo.)

[bcmills]

(CC @dsnet, who has been cleaning up time parsing lately)

[bcmills]

Looking at the layout documentation in https://pkg.go.dev/time#pkg-constants, I don't see any description of what actual format the MST abbreviation is supposed to match, but it seems that this case was added explicitly in CL 12922043 (CC @robpike), so presumably it should be fixed to use the correct offset instead of treating GMT+2 as an error.

[jwebb]

And another case - numeric offsets without the GMT prefix work differently again: time.Parse("2006-01-02 15:04:05 MST", "2022-08-24 00:00:00 +02") is accepted as valid but the +02 is treated the same as XXX, with no offset recorded. Also, +2 is surprisingly not accepted.

So to expand this issue: +0200 (using Z0700 format), and GMT+2 and +02 (using MST format) all ought logically to do the same thing, but in fact do three different things.