time: incomplete timezone coverage in tzdata

[jankampherbeek]

Go version

go 1.23.4 Darwin/macOs Sequoia 15.2, Mac mini with M$ processor

Output of go env in your module/workspace:

Last login: Thu Jan 23 19:45:31 on console
jankampherbeek@Mac-mini-van-Jan ~ % go env 
GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/jankampherbeek/Library/Caches/go-build'
GOENV='/Users/jankampherbeek/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/jankampherbeek/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/jankampherbeek/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.23.4/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.23.4/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.4'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/jankampherbeek/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='cc'
CXX='c++'
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 -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/ww/nt70fd0d3sz8d85b2t874x8h0000gn/T/go-build1452302060=/tmp/go-build -gno-record-gcc-switches -fno-common'
jankampherbeek@Mac-mini-van-Jan ~ %

What did you do?

The tzdata library is supposed to cover all data in the IANA tz Database.

However it appears that a part of the data is missing, for instance the data for the Netherlands before 1941.

I tested this with the following code:

package main

import (
    "fmt"
    "runtime"
    "time"
    _ "time/tzdata"
)

func main() {
    fmt.Printf("Go version: %s\n", runtime.Version())

    loc, err := time.LoadLocation("Europe/Amsterdam")
    if err != nil {
       panic(err)
    }

    dates := []time.Time{
       time.Date(1892, 1, 1, 12, 0, 0, 0, time.UTC),
       time.Date(1900, 1, 1, 12, 0, 0, 0, time.UTC),
       time.Date(1909, 1, 1, 12, 0, 0, 0, time.UTC),
       time.Date(1937, 1, 1, 12, 0, 0, 0, time.UTC),
       time.Date(1940, 5, 15, 12, 0, 0, 0, time.UTC),
       time.Date(1940, 5, 16, 12, 0, 0, 0, time.UTC),
       time.Date(1941, 5, 17, 12, 0, 0, 0, time.UTC),
    }

    for _, date := range dates {
       local := date.In(loc)
       name, offset := local.Zone()
       fmt.Printf("%v: %s (offset: %d seconds)\n",
          date.Format("2006-01-02"),
          name,
          offset)
    }
}

What did you see happen?

This code results in the following output:

Go version: go1.23.4
1892-01-01: BMT (offset: 1050 seconds)
1900-01-01: WET (offset: 0 seconds)
1909-01-01: WET (offset: 0 seconds)
1937-01-01: WET (offset: 0 seconds)
1940-05-15: WEST (offset: 3600 seconds)
1940-05-16: WEST (offset: 3600 seconds)
1941-05-17: CEST (offset: 7200 seconds)

What did you expect to see?

The data for the Netherlands for this period are in the tz database file 'backzone':

Rule Neth 1916 only - May 1 0:00 1:00 NST # Netherlands Summer Time
Rule Neth 1916 only - Oct 1 0:00 0 AMT # Amsterdam Mean Time
Rule Neth 1917 only - Apr 16 2:00s 1:00 NST
Rule Neth 1917 only - Sep 17 2:00s 0 AMT
Rule Neth 1918 1921 - Apr Mon>=1 2:00s 1:00 NST
Rule Neth 1918 1921 - Sep lastMon 2:00s 0 AMT
Rule Neth 1922 only - Mar lastSun 2:00s 1:00 NST
Rule Neth 1922 1936 - Oct Sun>=2 2:00s 0 AMT
Rule Neth 1923 only - Jun Fri>=1 2:00s 1:00 NST
Rule Neth 1924 only - Mar lastSun 2:00s 1:00 NST
Rule Neth 1925 only - Jun Fri>=1 2:00s 1:00 NST

# From 1926 through 1939 DST began 05-15, except that it was delayed by a week
# in years when 05-15 fell in the Pentecost weekend.

Rule Neth 1926 1931 - May 15 2:00s 1:00 NST
Rule Neth 1932 only - May 22 2:00s 1:00 NST
Rule Neth 1933 1936 - May 15 2:00s 1:00 NST
Rule Neth 1937 only - May 22 2:00s 1:00 NST
Rule Neth 1937 only - Jul 1 0:00 1:00 S
Rule Neth 1937 1939 - Oct Sun>=2 2:00s 0 -
Rule Neth 1938 1939 - May 15 2:00s 1:00 S
Rule Neth 1945 only - Apr 2 2:00s 1:00 S
Rule Neth 1945 only - Sep 16 2:00s 0 -
#STDOFF 0:19:32.13
Zone Europe/Amsterdam 0:19:32 - LMT 1835
0:19:32 Neth %s 1937 Jul 1
0:20 Neth +0020/+0120 1940 May 16 0:00
1:00 C-Eur CE%sT 1945 Apr 2 2:00
1:00 Neth CE%sT 1977
1:00 EU CE%sT

The correct value for the offset should have been 19 minutes and 32 seconds (1172 seconds). Is it possible to add the missing data to the tzdata library?

[gabyhelp]

Related Issues

• time: LoadLocationFromTZData with slim tzdata uses wrong cached zone #42216 (closed)
• time: Date returns a Time object with incorrect offset for years 1900-1905 #30099 (closed)
• time: time zone lookup using extend string makes wrong start time for non-DST zones #58682 (closed)
• time: Location interprets wrong timezone (DST) with slim zoneinfo [1.15 backport] #42138 (closed)
• time: time.LoadLocation reports unknown time zone for European Summer Time #69549 (closed)

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

[ianlancetaylor]

By default a Go program will read the tzdata files that are installed on your system. When I run your program on my Linux system, I get

Go version: go1.23.3
1892-01-01: AMT (offset: 1172 seconds)
1900-01-01: AMT (offset: 1172 seconds)
1909-01-01: AMT (offset: 1172 seconds)
1937-01-01: AMT (offset: 1172 seconds)
1940-05-15: +0020 (offset: 1200 seconds)
1940-05-16: CEST (offset: 7200 seconds)
1941-05-17: CEST (offset: 7200 seconds)

So my first guess would be that the problem is with the tzdata files installed on your system.

[jankampherbeek]

Thanks for your reply. You resuls are correct! But if I understand correctly, the tzdate library should include the whole tz database. The example code that I included is running perfectly for the other timezones that I tested and I do not use the tz datafiles. So there are timezones available in the tzdata library.

[ianlancetaylor]

All I can say for sure is that Go will look at the tzdata files on your system and use those. There are various different ways to build and distribute the tzdata files. It's entirely possible that my Linux distro made one choice and your macOS system made a different one.

According to https://data.iana.org/time-zones/tzdb-2022g/NEWS if tzdata is built with certain PACKRATLIST and PACKRATDATA options then old timezone information for Europe/Amsterdam, among others, will not be included. According to https://www.ubuntuupdates.org/package/core/focal/main/updates/tzdata the Ubuntu tzdata files are built with PACKRATLIST=zone.tab PACKRATDATA=backzone.

I think it's a reasonable assumption that the tzdata files on your system were built differently.

Closing as I don't think there is anything we can change in Go to fix this. Sorry for the trouble.

[jankampherbeek]

Thanks for you reply. I will give it a try with the parameters you mentioned.