When displaying the current time in an app, we often use timezone abbreviations like “JST” (Japan Standard Time) or “EST” (Eastern Standard Time). However, the truth is that there is no single, strictly unified global standard for these abbreviations.
To put it simply, they are just names used customarily by various regions and communities, and overlaps even exist. For example, the exact same abbreviation “CST” can actually refer to several different timezones (and their formal English names) as shown below:
- Central Standard Time (USA: UTC-6)
- China Standard Time (China: UTC+8)
- Cuba Standard Time (Cuba: UTC-5)
In this way, even for regions with completely different time differences, taking the initials causes them all to collide as “CST” on the system.
Recent Library Trends: From Abbreviations to Offsets
Partly due to this ambiguity, recent programming languages and standard libraries increasingly return simple time differences (offsets) like “GMT+9” or “UTC-05:00” even when you try to get an abbreviation (the exact format varies depending on the options and libraries).
For instance, let’s look at a code snippet using JavaScript’s standard Intl.DateTimeFormat to get a short timezone name (timeZoneName: 'short').
// Helper function to get the short name of a specified timezone
function getShortTzName(ianaTimeZone) {
const formatter = new Intl.DateTimeFormat('en-US', {
timeZone: ianaTimeZone,
timeZoneName: 'short'
});
const parts = formatter.formatToParts(new Date());
return parts.find(part => part.type === 'timeZoneName').value;
}
// Pattern 1: A case that used to return an alphabet abbreviation as expected
// (*In recent browsers, even specifying Tokyo often returns "GMT+9")
console.log(getShortTzName('Asia/Tokyo'));
// Output example: "JST" or "GMT+9"
// Pattern 2: Returns a GMT offset instead of an abbreviation
console.log(getShortTzName('Asia/Dubai'));
// Output: "GMT+4" (Ideally, we would expect something like "GST" here)
As you can see, despite being called with the exact same settings (locale and format options), depending on the specified city (timezone), it might return a familiar abbreviation like “JST”, or it might just return an offset like “GMT+4”.
*Update: I feel like specifying Asia/Tokyo used to return “JST” regardless of the locale. However, in recent browsers, perhaps prioritizing stricter display formats, it has become increasingly common for even Tokyo to return “GMT+9”. It is becoming extremely difficult to reliably obtain alphabet abbreviations using standard features alone.
While this is a consideration by the library to ensure system accuracy (a safe fallback), the reality for us developers is that consistently displaying user-friendly abbreviations like “EST” or “JST” in the app UI has become extremely difficult using standard features alone.
Why Can’t We Manage Time Using Only Names Like “EST”?
When developing time difference calculator apps, you might receive requests from users regarding the app’s addable city database, asking to “register and select timezone names like ‘EST (Eastern Standard Time)’, not just city names.” It’s an intuitive and very natural request.
Technically, it is entirely possible to register “EST” in the database as a simple, fixed offset (UTC-5). However, considering the purpose of an app like a world clock—to “know the exact time in a specific location”—managing and providing time using only the name “EST” is inappropriate. The biggest reason for this is the existence of Daylight Saving Time (DST).
Let’s look at a concrete example. Suppose you want to know the time in New York, USA. During the winter, New York observes “EST” (Eastern Standard Time), but in the summer, the clocks move forward, switching to “EDT” (Eastern Daylight Time).
If the app were to add and save “EST” as a fixed timezone just as the user requested, the offset wouldn’t change even when summer arrives, remaining at “EST” and causing a discrepancy with the actual current time in New York that the user wants to know.
To make matters worse, even within the same “EST” umbrella, the rules for when to switch between EST and EDT vary by city and region, and some regions don’t switch at all. For instance, Jamaica and Panama observe UTC-5 (EST) year-round and do not use Daylight Saving Time.
The main purpose of a time difference calculation app is to “know exactly what time it is right now in a certain place,” right? However, with only the instruction to “add as EST,” the system cannot determine whether the user wants the time in “New York (which has DST)” or “Jamaica (which does not).” As a result, the app fails to fulfill its primary purpose of presenting the correct time.
Going a step further, there is also the issue of “changes to rules in the past or future.” Depending on the country or region, they might suddenly abolish DST starting from a certain year or even change their base standard time offset.
In short, if you want to know the “accurate time throughout the year for a specific location (including accurate historical and future times),” you must base it on a city or region name (IANA timezone database name) like “America/New_York,” not a timezone abbreviation. Only after the city name is determined can the system accurately evaluate whether it’s summer so it should be EDT, winter so it should be EST, or “this region has no DST, so it remains EST.”
Implementation Example: Custom Abbreviation Determination Code
Since standard libraries don’t consistently return friendly abbreviations, if you want to display commonly used timezone abbreviations without relying on the environment, you need an approach where the app determines the abbreviation on its own.
Here, I’ll introduce a simplified JavaScript code example that creates a “key” combining the IANA timezone name and the current offset, and derives the accurate abbreviation from it.
/**
* Determine the abbreviation from the timezone name (IANA) and current offset.
* * offsetMinutes aligns with JavaScript's getTimezoneOffset() specification:
* negative for regions ahead of UTC, positive for regions behind.
*/
function getCustomTimezoneAbbr(ianaTimeZone, offsetMinutes) {
// Create a unique key by combining the timezone name and offset (in minutes)
const key = `${ianaTimeZone}_${offsetMinutes}`;
// Custom mapping dictionary (expand as needed)
const abbrMap = {
'Asia/Tokyo_-540': 'JST', // UTC+9
'America/New_York_300': 'EST', // UTC-5 (Winter: Standard Time)
'America/New_York_240': 'EDT', // UTC-4 (Summer: DST)
'Europe/London_0': 'GMT', // UTC+/-0 (Winter: Standard Time)
'Europe/London_-60': 'BST' // UTC+1 (Summer: DST)
};
// Return the abbreviation if it exists in the map; otherwise, return a generic GMT offset
if (abbrMap[key]) {
return abbrMap[key];
} else {
const sign = offsetMinutes > 0 ? '-' : '+';
const hours = Math.floor(Math.abs(offsetMinutes) / 60);
return `GMT${sign}${hours.toString().padStart(2, '0')}:00`;
}
}
// Execution example (New York in Winter)
console.log(getCustomTimezoneAbbr('America/New_York', 300)); // Outputs: "EST"
// Execution example (New York in Summer)
console.log(getCustomTimezoneAbbr('America/New_York', 240)); // Outputs: "EDT"
With this method, even if the same New York is specified, you can accurately output different abbreviations according to the season: “EST if the current offset is -5 hours (300 minutes)” and “EDT if it is -4 hours (240 minutes).”
Creating a custom mapping dictionary for all combinations of timezones and offsets is quite a daunting task. However, given that there is no global standard for timezone abbreviations and standard libraries do not fully support them, the reality is that we just have to power through with this kind of approach for now.
Timezone handling is profound and deeply intertwined with historical backgrounds like rule changes. Managing time internally with “cities (IANA timezone names)” while using custom mappings in the UI to stay user-friendly—this approach is the key to greatly improving your app’s usability.