Random Time Zone Generator

How this random time zone generator works

Press Generate and you get a time zone name with its abbreviation. Turn up the Number box for several at once, no repeats. Copy takes them to your clipboard. The three filters trim before the draw.

Central European Time. Moscow Time. Nepal Time. The familiar ones and a great many you have never seen.

Those abbreviations are the one part of the whole system that nobody ever standardised, and once you know that, you will never write "3pm IST" in an email again.

IST means three different things

Three different places on three different continents use it.

• Indian Standard Time, five and a half hours ahead of UTC.
• Irish Standard Time, one hour ahead, observed in the Irish summer.
• Israel Standard Time, two hours ahead.

Three countries, three offsets, one abbreviation. A meeting invitation that says "3pm IST" is not a meeting invitation. It is a coin toss.

It gets worse. PST is Pacific Standard Time, Pakistan Standard Time, and Pitcairn Standard Time. BST is British Summer Time, Bangladesh Standard Time, and Bougainville Time. CST is Central Standard Time in North America, China Standard Time, and Cuba Standard Time.

And the Irish one carries a joke inside it. In Irish law, the time observed in summer is the one legally designated as standard time. So the S in Irish Standard Time means standard, while the S in British Summer Time means summer, and the two countries are on the same clock.

Abbreviations are not a standard, and never were

Nobody issued these. There is no ISO number for time zone abbreviations, no committee, no register. They grew out of usage, and different countries invented their own for the same offsets.

The documentation for the IANA time zone database, which is the closest thing the world has to an authority on this, says the abbreviations are ambiguous in practice, gives CST and IST as its examples, and notes that some of the abbreviations in the database were invented by its own contributors because the place in question had no traditional English abbreviation at all.

Read that again. The people who keep the world's time zone data are telling you that some of the abbreviations they published were made up, and that you should not use any of them as identifiers.

That is not a criticism of them. It is honesty about a domain where honesty is scarce.

The database your phone actually uses

When your phone works out that a calendar invite from Berlin lands at the right hour in Kathmandu, it is not looking up CET or NPT. It is looking up something that looks like this:

Europe/Berlin. Asia/Kathmandu. America/New_York.

That is the IANA time zone database, also called tzdata, zoneinfo, or the Olson database after Arthur David Olson, who started it. Paul Eggert has edited it since 2005. ICANN took on responsibility for it in 2011.

The naming scheme is deliberate. Zones are named after a region and a city, never after a country, because countries change their names and their borders and cities mostly do not. Usually the largest city in the zone gets the name, and where two cities have shared a clock since 1970, only one of them appears.

A time zone, it turns out, is not a slice of the globe. It is a set of places that happen to have made the same political decisions about clocks for the last fifty years.

The maintainers say it is not authoritative

Buried in the tz database's own documentation is a warning that ought to be printed on the wall of every software company: the database is not authoritative, and it certainly contains errors.

They mean it. Their notes explain that predictions about the future become wrong when a government changes the law, that the record before 1970 is incomplete, and that the database does not attempt to record exact transition times for every place in every year.

Civil time is made by parliaments, recorded by whoever happened to be writing things down, and implemented by software written decades later. Any database that models it is modelling politics, not physics.

Which is the whole lesson. A list of abbreviations is a convenience. It is not a source of truth about what time it is anywhere. If you are writing code, use an offset or an IANA identifier. Use the abbreviation for the quiz.

The zones that are forty-five minutes off

Almost everybody assumes time zones are whole hours apart. They are not.

India runs five and a half hours ahead of UTC. So does Sri Lanka. Iran, Afghanistan and Myanmar all sit on half-hour offsets.

And then there is Nepal, which runs five hours and forty-five minutes ahead of UTC. Not five, not five and a half. Forty-five minutes past the half hour, a fifteen-minute offset from its enormous neighbour.

Nepal Time is NPT. In the database your phone uses, it is Asia/Kathmandu, and it exists because a country decided its clocks should be its own.

If you have ever wondered why scheduling software refuses to let you pick a time zone by dragging a slider, this is why. The world does not divide into twenty-four equal pieces, and it never has.

Ways people actually use this

• Testing software against reality. Generate ten and try them in your date parser. If it accepts IST without asking which one, you have found a bug.
• Quizzes and geography rounds. Show the abbreviation, ask for the place. CST is a trap. That is the point of asking it.
• Learning that the map lies. Generate twenty and look at how few of them are whole hours from anything.
• Naming things. Time zone abbreviations make decent codenames, and unlike most codenames they mean something.
• Settling an argument about meeting invites. Send whoever wrote "3pm IST" a link to this page.

Getting more out of the filters

• Type (IST) into Contains and every zone sharing that abbreviation comes back at once. Do the same with (PST) and (BST). It is a two-second demonstration of the whole problem.
• Type Standard into Contains. Almost every zone that comes back has a summer twin that is not called Standard.
• Ends with reads the last character of the entry, which here is the closing bracket around the abbreviation. Use Contains instead.
• Starts with matches the full zone name, so C gives you Central European Time and Coordinated Universal Time rather than everything abbreviated with a C.

Questions people ask

What does IST stand for?

India, Ireland or Israel, depending on who is speaking. That ambiguity is not a quirk. It is documented in the IANA time zone database's own notes as a reason not to use abbreviations at all.

How many time zones are there?

It depends entirely on what you count. Roughly forty distinct UTC offsets are in use, but the IANA database defines several hundred zones, because two places on the same offset today may have disagreed in 1974.

Is UTC the same as GMT?

For most everyday purposes, yes, and they name the same offset. They are not the same thing: UTC is a time standard maintained by atomic clocks, and GMT is a time zone. Software should say UTC.

Why is Nepal forty-five minutes off?

Because it chose to be. There is no geographical necessity behind any offset. Nepal Time is UTC plus five hours forty-five minutes, fifteen minutes ahead of Indian Standard Time.

Should I use these abbreviations in my code?

No. Use an IANA identifier like Asia/Kathmandu, or a numeric offset. The maintainers of the time zone database say so explicitly, and they know more about this than anyone.

References

1. Theory and pragmatics of the tz code and data
2. Sources for time zone and daylight saving time data
3. Tz database