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time: parseTimeZone() doesn't work for for many Zones formatted by Time.Format() #24071

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MyChaOS87 opened this issue Feb 23, 2018 · 4 comments
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time: parseTimeZone() doesn't work for for many Zones formatted by Time.Format() #24071

MyChaOS87 opened this issue Feb 23, 2018 · 4 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.11

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@MyChaOS87
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Please answer these questions before submitting your issue. Thanks!

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

1.9.4

Does this issue reproduce with the latest release?

yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/kasch/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/kasch/go"
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build708812266=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"time"
)

func main() {
	loc, err := time.LoadLocation("Europe/Istanbul")
	if err != nil {
		panic("Cannot load Timezone: " + err.Error())
	}

	timeStamp := time.Date(2018, 02, 23, 0, 0, 0, 0, loc)

	layout := "2006-01-02 15:04:05 MST"
	_, err = time.Parse(layout, timeStamp.Format(layout))

	if err != nil {
		panic("Cannot Parse Time: " + err.Error())
	}
}

Example in Go Playground
some Timezones from IANA database do get parsed correctly (eg. Europe/Istanbul)
see tz database +03 is used as timezone: https://github.com/eggert/tz/blob/6df98f4244eaa5ef9af50d71020faa9364b52f09/europe#L3738

there are more Timezones using times Like this one

What did you expect to see?

no panic, as I assume that the time package should be able to read every timestamp it produces with the same layout string

What did you see instead?

panic: Cannot Parse Time: parsing time "2018-02-23 00:00:00 +03" as "2006-01-02 15:04:05 MST": cannot parse "+03" as "MST"
@ianlancetaylor ianlancetaylor changed the title time.parseTimeZone() doesn't work for for many Zones formatted by Time.Format() time: parseTimeZone() doesn't work for for many Zones formatted by Time.Format() Feb 23, 2018
@ianlancetaylor
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The timezone handling by the time package is entirely dependent on the files supplied by your operating system. As you have shown, due to changes in the way that Turkey handles time zones, the files on your system print some dates in the Europe/Istanbul timezone using +03, but there is no timezone with that abbreviation. As the docs for time.Parse say, using MST in the parse string only matches defined timezones, and there is no timezone named +03 on your system.

It is not a goal that time.Time.Format and time.Parse be exact reverses of each other. In general, that is impossible, as is demonstrated by this case.

Use time.ParseInLocation instead.

Closing because there is nothing we can do.

@MyChaOS87
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Actually that is not completely true, as it can be matched on the system and the error is generated before trying to match it.

time.Parse calls time.ParseTimeZone() which then rejects this as it does not fit into the scheme having at least three upper case letters and a T in the end, see:
https://github.com/golang/go/blob/master/src/time/format.go#L1106-L1146

adding a third special case like:
if value[:1] == "+" || value[:1] == "-" { return 3, true }

and packing my example into a test in that package shows that this is easily solvable.

I am also willing to fix that using a bit more sophisticated special case then shown above, if this bug gets accepted.

@ianlancetaylor
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OK, sure, let's try that.

@gopherbot
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Change https://golang.org/cl/98157 mentions this issue: time: add support for parsing timezones denoted by sign and offset

@andybons andybons added the NeedsFix The path to resolution is known, but the work has not been done. label Mar 7, 2018
@andybons andybons added this to the Go1.11 milestone Mar 7, 2018
@ALTree ALTree mentioned this issue Jun 24, 2018
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@golang golang locked and limited conversation to collaborators Mar 8, 2019
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