time.Equal is false after MarshalJSON/UnmarshalJSON outside UTC; affected/package: time

[dreisBHT]

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

$ go version
go version go1.21.4 linux/amd64

Does this issue reproduce with the latest release?

Yes, go1.21.4 is the latest as far as I can tell.

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

go env Output

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/XXX/.cache/go-build'
GOENV='/home/rXXX/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/XXX/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/XXX/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.4'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build517579625=/tmp/go-build -gno-record-gcc-switches'

What did you do?

https://go.dev/play/p/X1yF2C5EqyJ
Note the issue does not show in Go Playground because it is located in the UTC time zone.

package main

import (
	"fmt"
	"time"
)

func main() {
	t1 := time.Time{}.Local()

	b, _ := t1.MarshalJSON()

	var t2 time.Time
	t2.UnmarshalJSON(b)

	fmt.Printf("%v\n", t1.Equal(t2))
}

What did you expect to see?

true

What did you see instead?

false

Notes:

I am aware of #12141, #10089, #17875. All of them seem to be discussing time.Equal vs == but in my case the actual time instant is different:
t1.wall is 0, t1.ext is 0
t2.wall is 0, t2.ext is 44

t1.IsZero() is true
t2.IsZero() is false

t1.String() gives "0001-01-01 00:57:44 +0057 LMT"
t2.String() gives "0001-01-01 00:57:44 +0057 +0057"

The strange 57:44 offset is, as far as I can tell, because time before time zones is converted to the local solar time. So this issue shows up only if you are located outside of UTC. I assume those weird fractional time zones are expected, based on this Stack overflow issue:
https://dba.stackexchange.com/questions/127965/why-does-time-zone-have-such-a-crazy-offset-from-utc-on-year-0001-in-postgres

[seankhliao]

I believe this is working as intended, and the zero time is not expected to be valid

[dreisBHT]

You will have the same issue with any time before circa 1880. I used zero time here simply because it illustrates the issue even more clearly.

I just tried with:
t1 := time.Date(100, 2, 3, 11, 21, 33, 0, time.Local),
same issue.

I would also question why is midnight 1st Jan year 1 so special that it is "not valid" but that is perhaps beside the point.

For completeness sake:

	t1 := time.Date(1492, 10, 12, 8, 47, 59, 0, time.Local)

	b, _ := t1.MarshalJSON()

	var t2 time.Time
	t2.UnmarshalJSON(b)

	fmt.Printf("%v\n", t1.String())
	fmt.Printf("%v\n", t2.String())
	fmt.Printf("%v\n", t1.Equal(t2))

Prints (in my case):
1492-10-12 08:47:59 +0057 LMT
1492-10-12 08:47:59 +0057 +0057
false

[seankhliao]

Duplicate of #57040

[defwxyz]

The following test is ok now and show the importance of using .UTC()

func TestTime(t *testing.T) {

	now1 := time.Now().UTC()
	n1, _ := now1.MarshalJSON()
	time.Sleep(1 * time.Second)
	now2 := time.Time{}.UTC()
	now2.UnmarshalJSON(n1)
	n2, _ := now2.MarshalJSON()
	// is ok
	if string(n1) != string(n2) {
		t.Errorf("time marshall unmarshall is not identity: %v != %v", string(n1), string(n2))
	}
	// is ok
	if !now1.Equal(now2) {
		t.Error("time.Equal return false")
	}
	// is ok
	if now1.String() != now2.String() {
		t.Errorf("time marshall unmarshall is not identity:\nnow1=%v\n!=\nnow2=%v\n", now1, now2)
	}
	// is ok
	if now1 != now2 {
		t.Errorf("time marshall unmarshall is not identity:\nnow1=%v\n!=\nnow2=%v\n", now1, now2)
	}
}