Setting timezone globally in golang

I may be late but setting timezone in a global env is not a reliable approach. It should be set globally in a variable or in a struct. The below is an example of timezone set in a variable. Also in Go Playground

package main

import (
    "fmt"
    "log"
    "time"
)

func main() {
    if err := setTimezone("America/Los_Angeles"); err != nil {
        log.Fatal(err) // most likely timezone not loaded in Docker OS
    }
    t := getTime(time.Now())
    fmt.Println(t)
}

var loc *time.Location

func setTimezone(tz string) error {
    location, err := time.LoadLocation(tz)
    if err != nil {
        return err
    }
    loc = location
    return nil
}

func getTime(t time.Time) time.Time {
    return t.In(loc)
}

Adding my anser here for people who stumbled on this page. There's a global variable in time package, use it like this in main.go

package main

import "time"

func main() {
    loc, err := time.LoadLocation("Africa/Cairo")
    // handle err
    time.Local = loc // -> this is setting the global timezone
}

Your system must have timezone database installed. In docker, you must apt get/apk add tzdata. But if you're using go1.15, you can also embed the timezone database without installing tzdata on system.

package main

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

func main() {
    loc, err := time.LoadLocation("Africa/Cairo")
    // handle err
    time.Local = loc // -> this is setting the global timezone
}

You can achieve what you want from inside your app using os.Setenv("TZ", "Africa/Cairo"), what matters is that you must call this before any other package uses anything from the time package.

How to ensure that? Create a package that does nothing else except sets the timezone (later you may add other things to it, but for our example that's enough).

Like this:

package tzinit

import (
    "os"
)

func init() {
    os.Setenv("TZ", "Africa/Cairo")
}

Import this tzinit package first thing in your main package like this:

package main

import _ "path/to/tzinit"

// Your other, "regular" imports:
import (
    "fmt"
    "os"
    "time"
    ...
)

And so setting the TZ env var will happen before any other package could access the time package.

Note that I used a separate import declaration just for tzinit, and the reason for this is because many code editors / IDEs will rearrange your imports alphabetically, this will ensure that importing tzinit will remain the first import.

A word of warning.

The Spec: Package initialization states the requirements and rules of initializing packages, and the order in which imports are processed is not specified (only thing guaranteed is that all referenced package will be initialized recursively before it can be used). This means that although current compilers process them as listed, you cannot rely on this for 100%. There's also the issue of having multiple source files even for the main package, supplying them in different order to the compiler may also change the initialization order. The spec has this as a "recommendation":

To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler.

So to be on the safe side, best would be to set the TZ environment variable before the Go app is launched.