Golang : Display list of time zones with GMT

For this tutorial, we will learn how to grab the list of time zones found in Unix/Darwin(Mac)/Linux/Windows and display the list complete with GMT information. Knowing the differences in time zones can be useful in calculating the time at different locations.

NOTES:

Golang has pretty good support for time zones, but on Windows platform it can be a pain as the time zones names cannot be used directly in Golang. You will need to convert the given time zone on Windows platform to equivalent location names found in *nix platforms in order to use the time.LoadLocation().

---

For Windows, see https://msdn.microsoft.com/en-US/library/system.timezoneinfo.findsystemtimezonebyid(v=vs.110).aspx

Under the Remarks

FindSystemTimeZoneById tries to match id to the subkey names of the HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Time Zones branch of the registry under Windows XP and Windows Vista. This branch does not necessarily contain a comprehensive list of time zone identifiers.....

See also https://golang.org/src/time/zoneinfo_windows.go line 59 to 79

Here you go!

---

 package main

 import (
  "bytes"
  "fmt"
  "io/ioutil"
  "os"
  "os/exec"
  "regexp/syntax"
  "runtime"
  "strings"
  "time"
  "unicode/utf8"
  // "golang.org/x/sys/windows/registry"
 )

 // zoneDirs adapted from https://golang.org/src/time/zoneinfo_unix.go

 // https://golang.org/doc/install/source#environment
 // list of available GOOS as of 10th Feb 2017
 // android, darwin, dragonfly, freebsd, linux, netbsd, openbsd, plan9, solaris,windows

 var zoneDirs = map[string]string{
  "android": "/system/usr/share/zoneinfo/",
  "darwin": "/usr/share/zoneinfo/",
  "dragonfly": "/usr/share/zoneinfo/",
  "freebsd": "/usr/share/zoneinfo/",
  "linux": "/usr/share/zoneinfo/",
  "netbsd": "/usr/share/zoneinfo/",
  "openbsd": "/usr/share/zoneinfo/",
  // "plan9":"/adm/timezone/", -- no way to test this platform
  "solaris": "/usr/share/lib/zoneinfo/",
  "windows": `SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\`,
 }

 var zoneDir string

 var timeZones []string

 // InSlice ... check if an element is inside a slice
 func InSlice(str string, list []string) bool {
  for _, v := range list {
 if v == str {
 return true
 }
  }
  return false
 }

 // ReadTZFile ... read timezone file and append into timeZones slice
 func ReadTZFile(path string) {
  files, _ := ioutil.ReadDir(zoneDir + path)
  for _, f := range files {
 if f.Name() != strings.ToUpper(f.Name()[:1])+f.Name()[1:] {
 continue
 }
 if f.IsDir() {
 ReadTZFile(path + "/" + f.Name())
 } else {
 tz := (path + "/" + f.Name())[1:]
 // check if tz is already in timeZones slice
 // append if not
 if !InSlice(tz, timeZones) { // need a more efficient method...

 // convert string to rune
 tzRune, _ := utf8.DecodeRuneInString(tz[:1])

 if syntax.IsWordChar(tzRune) { // filter out entry that does not start with A-Za-z such as +VERSION
 timeZones = append(timeZones, tz)
 }
 }
 }
  }

 }

 func ListTimeZones() {
  if runtime.GOOS == "nacl" || runtime.GOOS == "" {
 fmt.Println("Unsupported platform")
 os.Exit(0)
  }

  // detect OS
  fmt.Println("Time zones available for : ", runtime.GOOS)
  fmt.Println("------------------------")

  fmt.Println("Retrieving time zones from : ", zoneDirs[runtime.GOOS])

  if runtime.GOOS != "windows" {
 for _, zoneDir = range zoneDirs {
 ReadTZFile("")
 }
  } else { // let's handle Windows
 // if you're building this on darwin/linux
 // chances are you will encounter
 // undefined: registry in registry.OpenKey error message
 // uncomment below if compiling on Windows platform

 //k, err := registry.OpenKey(registry.LOCAL_MACHINE, `SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones`, registry.ENUMERATE_SUB_KEYS|registry.QUERY_VALUE)

 //if err != nil {
 // fmt.Println(err)
 //}
 //defer k.Close()

 //names, err := k.ReadSubKeyNames(-1)
 //if err != nil {
 // fmt.Println(err)
 //}

 //fmt.Println("Number of timezones : ", len(names))
 //for i := 0; i <= len(names)-1; i++ {
 // check if tz is already in timeZones slice
 // append if not
 // if !InSlice(names[i], timeZones) { // need a more efficient method...
 //  timeZones = append(timeZones, names[i])
 // }
 //}

 // UPDATE : Reading from registry is not reliable
 // better to parse output result by "tzutil /g" command
 // REMEMBER : There is no time difference between Coordinated Universal Time and Greenwich Mean Time ....
 cmd := exec.Command("tzutil", "/l")

 data, err := cmd.Output()

 if err != nil {
 panic(err)
 }

 fmt.Println("UTC is the same as GMT")
 fmt.Println("There is no time difference between Coordinated Universal Time and Greenwich Mean Time ....")
 GMTed := bytes.Replace(data, []byte("UTC"), []byte("GMT"), -1)

 fmt.Println(string(GMTed))

  }

  now := time.Now()

  for _, v := range timeZones {

 if runtime.GOOS != "windows" {

 location, err := time.LoadLocation(v)
 if err != nil {
 fmt.Println(err)
 }

 // extract the GMT
 t := now.In(location)
 t1 := fmt.Sprintf("%s", t.Format(time.RFC822Z))
 tArray := strings.Fields(t1)
 gmtTime := strings.Join(tArray[4:], "")
 hours := gmtTime[0:3]
 minutes := gmtTime[3:]

 gmt := "GMT" + fmt.Sprintf("%s:%s", hours, minutes)
 fmt.Println(gmt + " " + v)

 } else {
 fmt.Println(v)
 }

  }
  fmt.Println("Total timezone ids : ", len(timeZones))
 }

 func main() {

  ListTimeZones()

 }

---

Sample output on LinuxMint:

---

Time zones available for : linux

------------------------

Retrieving time zones from : /usr/share/zoneinfo/

GMT+00:00 Africa/Abidjan

GMT+00:00 Africa/Accra

GMT+03:00 Africa/Addis_Ababa

GMT+01:00 Africa/Algiers

GMT+03:00 Africa/Asmara

GMT+03:00 Africa/Asmera

GMT+00:00 Africa/Bamako

GMT+01:00 Africa/Bangui

GMT+00:00 Africa/Banjul

GMT+00:00 Africa/Bissau

GMT+02:00 Africa/Blantyre

GMT+01:00 Africa/Brazzaville

GMT+02:00 Africa/Bujumbura

GMT+02:00 Africa/Cairo

GMT+00:00 Africa/Casablanca

GMT+01:00 Africa/Ceuta

GMT+00:00 Africa/Conakry

GMT+00:00 Africa/Dakar

GMT+03:00 Africa/DaresSalaam

GMT+03:00 Africa/Djibouti

GMT+01:00 Africa/Douala

GMT+00:00 Africa/El_Aaiun

GMT+00:00 Africa/Freetown

GMT+02:00 Africa/Gaborone

GMT+02:00 Africa/Harare

GMT+02:00 Africa/Johannesburg ....

GMT+14:00 Pacific/Apia

GMT+13:00 Pacific/Auckland

GMT+11:00 Pacific/Bougainville

GMT+13:45 Pacific/Chatham

GMT+10:00 Pacific/Chuuk

GMT-05:00 Pacific/Easter

GMT+11:00 Pacific/Efate

GMT+13:00 Pacific/Enderbury

GMT+13:00 Pacific/Fakaofo

GMT+12:00 Pacific/Fiji

GMT+12:00 Pacific/Funafuti

GMT-06:00 Pacific/Galapagos

GMT-09:00 Pacific/Gambier

GMT+11:00 Pacific/Guadalcanal

GMT+10:00 Pacific/Guam

GMT-10:00 Pacific/Honolulu

GMT-10:00 Pacific/Johnston

GMT+14:00 Pacific/Kiritimati

GMT+11:00 Pacific/Kosrae

GMT+12:00 Pacific/Kwajalein

GMT+12:00 Pacific/Majuro

GMT-09:30 Pacific/Marquesas

GMT-11:00 Pacific/Midway

GMT+12:00 Pacific/Nauru

GMT-11:00 Pacific/Niue

GMT+11:00 Pacific/Norfolk

GMT+11:00 Pacific/Noumea

GMT-11:00 Pacific/Pago_Pago

GMT+09:00 Pacific/Palau

GMT-08:00 Pacific/Pitcairn

GMT+11:00 Pacific/Pohnpei

GMT+11:00 Pacific/Ponape

GMT+10:00 Pacific/Port_Moresby

GMT-10:00 Pacific/Rarotonga

GMT+10:00 Pacific/Saipan

GMT-11:00 Pacific/Samoa

GMT-10:00 Pacific/Tahiti

GMT+12:00 Pacific/Tarawa

GMT+13:00 Pacific/Tongatapu

GMT+10:00 Pacific/Truk

GMT+12:00 Pacific/Wake

GMT+12:00 Pacific/Wallis

GMT+10:00 Pacific/Yap

GMT+01:00 Poland

GMT+00:00 Portugal

GMT+08:00 ROC

GMT+09:00 ROK

GMT+08:00 Singapore

GMT-04:00 SystemV/AST4

GMT-04:00 SystemV/AST4ADT

GMT-06:00 SystemV/CST6

GMT-06:00 SystemV/CST6CDT

GMT-05:00 SystemV/EST5

GMT-05:00 SystemV/EST5EDT

GMT-10:00 SystemV/HST10

GMT-07:00 SystemV/MST7

GMT-07:00 SystemV/MST7MDT

GMT-08:00 SystemV/PST8

GMT-08:00 SystemV/PST8PDT

GMT-09:00 SystemV/YST9

GMT-09:00 SystemV/YST9YDT

GMT+03:00 Turkey

GMT+00:00 UCT

GMT-09:00 US/Alaska

GMT-10:00 US/Aleutian

GMT-07:00 US/Arizona

GMT-06:00 US/Central

GMT-05:00 US/East-Indiana

GMT-05:00 US/Eastern

GMT-10:00 US/Hawaii

GMT-06:00 US/Indiana-Starke

GMT-05:00 US/Michigan

GMT-07:00 US/Mountain

GMT-08:00 US/Pacific

GMT-08:00 US/Pacific-New

GMT-11:00 US/Samoa

GMT+00:00 UTC

GMT+00:00 Universal

GMT+03:00 W-SU

GMT+00:00 WET

GMT+00:00 Zulu

Total timezone ids : 606

---

References:

https://golang.org/src/time/zoneinfo_unix.go

https://technet.microsoft.com/en-us/library/cc749073(v=ws.10).aspx

https://www.socketloop.com/tutorials/golang-detect-os-operating-system

https://www.socketloop.com/references/golang-time-time-zone-function-example