time: support Decimal Comma for fractional seconds

[djaglowski]

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

$ go version
go version go1.15.3 darwin/amd64

Does this issue reproduce with the latest release?

Yes, I can reproduce it on the playground

What operating system and processor architecture are you using (go env)?

GOARCH=amd64
GOOS=darwin

go env Output

$ go env
GOARCH="amd64"
GOOS="darwin"
...

What did you do?

Attempt to parse a date/time string containing milliseconds preceded by a decimal comma. Example:
https://play.golang.org/p/wxEOUEVvgaV

What did you expect to see?

Time is 02/12/2019 15:45:48,746

What did you see instead?

parsing time "02/12/2019 15:45:48,746" as "02/01/2006 15:04:05,000": cannot parse "746" as ",000"

Additional Context

ISO_8601 states that decimal comma is a supported, even preferred, value for the separator between seconds and fractional seconds.

Related Issues:

• time: decimal comma not supported in fractional seconds #6189
• Go parse time error? #27746
• time: support parsing and formatting timestamps that don't use period for fractional seconds #26002
• proposal: time: support Colon for fractional seconds #36145
• time/format: Error parsing time format with European offset #43813

[rsc]

The proposal here is to accept 05,000 as the same as 05.000 in a time format (and similarly for other numbers of zeros).
Presumably dot would continue to only match dot, and comma would only match comma.
Do I have that right?

[rsc]

/cc @robpike

[djaglowski]

@rsc That's correct.

[robpike]

Seems reasonable on its own, although I am wary of what might follow. Internationalization was never the goal of this package.

[rsc]

It's definitely not a goal to do full internationalization, but ISO-8601 does recognize , as a decimal fraction separator, so at least we are following a defined standard for times. And it does not have any other internationalization-related changes, so that's a clear line we can refer to if people want other changes.

In particular, I don't think this opens us to internationalized month and day names. Those will stay just the English set.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group