time: Parse of am/pm should be forgiving of spaces and case

[spekary]

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

1.17.5

Does this issue reproduce with the latest release?

Yes

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

go env Output

Mac OS

What did you do?

import (
"testing"
"time"
)

func TestParse(t *testing.T) {
var tm time.Time
var err error

tm, err = time.Parse("3:04PM", "5:23 PM")
if err != nil {
	t.Error(err)
}
if tm.Hour() != 17 {
	t.Error("hour does not match")
}

tm, err = time.Parse("3:04pm", "5:23PM")
if err != nil {
	t.Error(err)
}
if tm.Hour() != 17 {
	t.Error("hour does not match")
}

}

What did you expect to see?

Tests pass

What did you see instead?

=== RUN TestParse
parse_test.go:14: parsing time "5:23 PM" as "3:04PM": cannot parse "M" as "PM"
parse_test.go:17: hour does not match
parse_test.go:22: parsing time "5:23PM" as "3:04pm": cannot parse "" as "pm"
parse_test.go:25: hour does not match
--- FAIL: TestParse (0.00s)

[spekary]

If this is too big a change, a possible solution would be an additional layout specifier: "Pm", or more wonky "pM". This could indicate a desire to be forgiving of spaces and case.

[thanm]

@rsc per owners.

It seems as though it would be very easy to write a bit of Go code that would preprocess time values to put them in canonical form prior to the time.Parse() invocation. That is, if you want both "PM" and "pm" to be accepted, just pattern match the tail end of the value string and replace it if needed.

[andlabs]

Another example, including periods/dots:

parsing time "07/13/2023 9:34 A.M." as "01/02/2006 3:04 P.M.": cannot parse "A.M." as " P.M."

(error message includes all relevant parameters; just tried this in the 1.20 playground)

[ghost]

The bottom of https://knowadays.com/blog/writing-the-time-am-and-pm-or-a-m-and-p-m/ has a summary of various style guides that suggests almost all combinations are possible and acceptable.