time: Parse formats allow ambiguity

[m-messiah]

Go version

go version go1.22.3 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE='on'
GOARCH='arm64'
GOOS='darwin'

What did you do?

Time Parse always assumes RFC3339 date and does not allow to change it.

For example https://go.dev/play/p/1gWqncWgd0_h:

a, err := time.Parse("2006-21-01", "2024-23-03")

expected to parse the date as 23 March 2024, but fails with error parsing time "2024-23-03" as "2006-21-01": cannot parse "-03" as "1"

Moreover, time.DateOnly format (as well as time.DateTime) assume RFC3339 with ambiguity to anyone who does not follow the RFC, as both 01 and 02 could be treated as Month

go/src/time/format.go

Lines 123 to 124 in 5bf8c0c

	DateTime = "2006-01-02 15:04:05"
	DateOnly = "2006-01-02"

What did you see happen?

https://go.dev/play/p/qnIFX3Q_zUL

1 January 1 parsing time "2024-23-03" as "2006-21-01": cannot parse "-03" as "1"
2006-01-02

What did you expect to see?

Expect time.Parse to follow the format provided without hidden assumptions leading to errors.
Expect time.DateOnly and time.DateTime to provide self-explanatory format without ambiguity.

[seankhliao]

That's the wrong layout to use. time.Parse correctly treated it as:

• 2006: 2004
• -: -
• 2: 23
• 1: -03, an error

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[m-messiah]

Ok, I reread the Layout comment and it has some handy things about Day and Month:

go/src/time/format.go

Lines 60 to 62 in 5bf8c0c

	// Month: "Jan" "January" "01" "1"
	// Day of the week: "Mon" "Monday"
	// Day of the month: "2" "_2" "02"

saying month is always 01 and day is always 02, meaning my playground could be rewritten as https://go.dev/play/p/CcPYcJ5rRPN

And it works, correctly returning the 23 of March, but go vet sends warning about such format after #48801.

[rsc]

Where did you encounter yyyy-dd-mm dates? When we accepted #48801, we believed they were exceedingly rare.