Why Does Adding a UTC Offset to UTC Time Not Accurately Produce Local Time in Go?

Issues Converting UTC to Local Time in Go

Problem Overview

When attempting to convert UTC time to local time, certain users encounter incorrect results. Specifically, the issue arises when adding a duration, calculated from a provided UTC offset, to the current UTC time. This returned value deviates from the anticipated local time.

Initial Approach

The code snippet initializes a map containing UTC offsets for various countries. It attempts to convert the offset for "Hungary" into a duration, adding it to the UTC time, and displays the result as "local time."

package main

import "fmt"
import "time"

func main() {

    m := make(map[string]string)
    m["Hungary"] = "+01.00h"

    offSet, err := time.ParseDuration(m["Hungary"])
    if err != nil {
        panic(err)
    }
    t := time.Now().UTC().Add(offSet)
    nice := t.Format("15:04")

    fmt.Println(nice)
}

Incorrect Results

Upon execution, the code produces incorrect local times, especially for countries with an offset from UTC. For instance, with Hungary's one-hour offset, the result doesn't reflect the actual local time.

Root Cause

The incorrect results stem from the assumption that the adjusted UTC time is equivalent to the local time. However, this assumption disregards the concept of time zones.

Time Zone Considerations

Time zones are geographical regions that observe a standard time for legal, commercial, and social purposes. Different time zones have their own offsets from UTC, representing the time difference from the prime meridian.

Correct Approach

To accurately convert UTC time to a specific local time, it's essential to consider the time zone associated with that location. The time.LoadLocation function allows you to obtain a time zone object based on its string identifier.

var countryTz = map[string]string{
    "Hungary": "Europe/Budapest",
    "Egypt":   "Africa/Cairo",
}

func timeIn(name string) time.Time {
    loc, err := time.LoadLocation(countryTz[name])
    if err != nil {
        panic(err)
    }
    return time.Now().In(loc)
}

func main() {
    utc := time.Now().UTC().Format("15:04")
    hun := timeIn("Hungary").Format("15:04")
    eg := timeIn("Egypt").Format("15:04")
    fmt.Println(utc, hun, eg)
}

This solution ensures that the converted time reflects the local time for the specified country, taking into account the relevant time zone.

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