Please answer these questions before submitting your issue. Thanks!

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

go version go1.10 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

GOARCH="amd64"
GOOS="darwin"

What did you do?

package main

import (
	"fmt"
	"time"
)

func main() {
	t, err := time.Parse(time.RFC3339Nano, "2018-04-14T15:22:24,859303000+00:00")
	if err != nil {
		panic(err)
	}
	fmt.Println(t.String())
}

What did you expect to see?

2018-04-14 15:22:24.859303 +0000 UTC

What did you see instead?

panic: parsing time "2018-04-14T15:22:24,859303000+00:00" as "2006-01-02T15:04:05.999999999Z07:00": cannot parse ",859303000+00:00" as "Z07:00"

I ran across this while I was combining a small go utility with some unix utilities. One of those utilities was GNU's date, which I used as follows:

date -u --iso-8601=ns
2018-04-14T15:36:21,837840000+00:00

I was surprised to see a comma (,) to denote the fractional seconds.

Now, interestingly enough, if I use --rfc-3339=ns, it returns a dot (.):

date -u --rfc-3339=ns
2018-04-14 15:38:42.023325000+00:00

I dug into the RFC3339 spec, and found that using a comma is perfectly valid:

Time:

   time-hour         = 2DIGIT ; 00-24
   time-minute       = 2DIGIT ; 00-59
   time-second       = 2DIGIT ; 00-58, 00-59, 00-60 based on
                              ; leap-second rules
+  time-fraction     = ("," / ".") 1*DIGIT
   time-numoffset    = ("+" / "-") time-hour [[":"] time-minute]
   time-zone         = "Z" / time-numoffset

   timeopt-hour      = "-" / (time-hour [":"])
   timeopt-minute    = "-" / (time-minute [":"])

   timespec-hour     = time-hour [[":"] time-minute [[":"] time-second]]
   timespec-minute   = timeopt-hour time-minute [[":"] time-second]
   timespec-second   = "-" timeopt-minute time-second
   timespec-base     = timespec-hour / timespec-minute / timespec-second

   time              = timespec-base [time-fraction] [time-zone]

   iso-date-time     = date "T" time

As I said, I can work around this, but I figured I create this issue to see if this is something that should be fixed in Go, or if not, and the issue is closed, anyone else running into this can at least find the issue and know what's going on 👍