time: RFC3339 only parses the subset of valid RFC3339 datetimes where `T` is used to separate the date & time

[hadmut]

Go version

go version go1.24.4 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/hadmut/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/hadmut/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1507873698=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/hadmut/gotest/go.mod'
GOMODCACHE='/home/hadmut/go/pkg/mod'
GONOPROXY='*.danisch.de'
GONOSUMDB='*.danisch.de'
GOOS='linux'
GOPATH='/home/hadmut/go'
GOPRIVATE='*.danisch.de'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go-1.24'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/hadmut/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go-1.24/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.4'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Run this code:

package main

import (
	"fmt"
	"time"
)

func main() {
	t, err := time.Parse(time.RFC3339, "2025-12-13 22:44:41+02:00")
	if err != nil {
		panic(err.Error())
	}
	fmt.Println(t)
}

(playground)

What did you see happen?

panic: parsing time "2025-12-13 22:44:41+02:00" as "2006-01-02T15:04:05Z07:00": cannot parse " 22:44:41+02:00" as "T"

goroutine 1 [running]:
main.main()
	/tmp/sandbox1968158324/prog.go:11 +0xac

Program exited.

What did you expect to see?

2025-12-13 22:44:41 +0200 +0200

[seankhliao]

Your date tool is wrong.
The T is required as per RFC 3339: https://datatracker.ietf.org/doc/html/rfc3339#section-5.6

[collinfunk]

@seankhliao I disagree. The section you link states:

The following profile of ISO 8601 [ISO8601] dates SHOULD be used in new protocols on the Internet. This is specified using the syntax description notation defined in [ABNF].

[...]
date-time = full-date "T" full-time
[...]

NOTE: ISO 8601 defines date and time separated by "T". Applications using this syntax may choose, for the sake of readability, to specify a full-date and full-time separated by (say) a space character.

So, ISO 8601 requires a T be used. RFC 3339 allows a space to be used in it's place.

This has come up quite a few times on GNU Coreutils mailing list:

date --rfc-3339=seconds
2025-12-13 18:03:43-08:00

[hadmut]

Yeah, both bug reports were submitted by me.

golang insists on "T",
coreutils insists, that " " is valid as well.

Wonderful example of ambiguous definition.

[Jorropo]

I've editorialized your bug report.
Here is some context I've removed:

The reason this is a problem is because the gnu core utils date tool will output RFC3339 with a instead of T in some cases like:

date --rfc-3339=seconds

---

I am not sure I read this paragraph properly:

      NOTE: ISO 8601 defines date and time separated by "T".
      Applications using this syntax may choose, for the sake of
      readability, to specify a full-date and full-time separated by
      (say) a space character.

Because the way I read it basically says that this line above:

 date-time       = full-date "T" full-time

Is just wrong and should be:

 date-time       = full-date ("T" / " ") full-time

But then why wouldn't the authors have just changed the date-time = ... line ?

[Jorropo]

cc @rsc per https://dev.golang.org/owners

[gabyhelp]

Related Issues

• time.RFC3339 does not support space between date and time #41461 (closed)
• time: layout string parser doesn't support all needed features to cover the RFC 3339 standard #63488 (closed)
• time.parse cann ot parse RFC3339 reference string (or any other I tried) #68527 (closed)
• Invalid example for RFC3339 time layout - Invalid according to RFC and also time.Parse fails #71377 (closed)
• time.RFC3339* cannot be parsed using time.RFC3339* as a format in time.Parse #38469 (closed)
• time: Weird behavior with RFC3339 parser #30341 (closed)
• time: time zone parsing when format includes time zone seconds fails #68263 (closed)
• time.RFC3339 does not even parse with time.Parse #21443 (closed)
• time: Parse of RFC3339 is incorrect for timezone field #54570 (closed)
• time: unexpected change in parseRFC3339 behaviour #55876 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[collinfunk]

@Jorropo I think this paragraph should clear it up. It comes right before the grammar:

The following profile of ISO 8601 [ISO8601] dates SHOULD be used in new protocols on the Internet. This is specified using the syntax description notation defined in [ABNF].

So the grammar in the RFC is for ISO 8601. It is not a complete grammar for ISO 8601, which is a long document. For example, it doesn't mention that fractional seconds should be seperated by a comma or period.