# How Many Time Zones Are There Anyway?

Nowadays, it’s an even bet that your newest project somehow connects to the Internet and, thus, to the world. Even if it doesn’t, if you share your plans, someone might reproduce your creation in some far distant locale. If your design uses time, you might need to think about time zones. Easy, right? That’s what [Zain Rizvi] thought until he tried to deploy something that converted between timezones. You can learn from his misconceptions thanks to a detailed post he provides.

You might think, “What’s the big deal?” After all, there’s UTC, and then there are 12 time zones ahead of UTC and 12 time zones later. But that’s not even close to true.

As [Zain] found out, there are 27 hours in a full-day cycle if you count UTC as one hour. Why? Because some islands in the Pacific wanted to be on the wrong side of the International Date Line. So there are a few extra zones to accommodate them.

You can’t even count on time zones being offset by an hour from the previous zone. Several zones have a half-hour offset from UTC (for example, India’s standard time is 5.5 hours from UTC). But surely the offset is always either a whole number or a number where the fractional part is 0.5, right?

Um, no. Nepal wants the sun to be directly over the mountain at noon, so it offsets by 45 minutes! [Zain] wonders — as we do — what would happen if the mountain shifted over time? Until 1940, Amsterdam used a 20-minute offset. Some cities are split with one half in one time zone and another in the other.

Of course, there are the usual problems with multiple names for each zone, both because many countries want their own zone and because the exact same zone is different in different languages. Having your own zone is not just for vanity, though. Daylight savings time rules will vary by zone and even, in some cases, only in certain parts of a zone. For example, in the United States, Arizona doesn’t change to daylight savings time. Oh, except for the Navajo Nation in Arizona, which does! Some areas observe daylight savings time that starts and ends multiple times during the year. Even if you observe daylight savings time, there are cases where the time shift isn’t an entire hour.

Besides multiple names, common names for zones often overlap. For example, in the United States, the Eastern Standard Time zone differs from Australia’s. Confused? You should be.

Maybe we should have more respect for multiple time zone clock projects. We’ve noticed these problems before when we felt sorry for the people who maintain the official time zone database.

## 34 thoughts on “How Many Time Zones Are There Anyway?”

1. Herr Brain says:

Hot take: time zones no longer need to exist. They served their purpose back when railroads started connecting cities that all ran at different times. But now we live in an age where clocks can be synchronized within a few milliseconds (or better) anywhere that the sky is visible, and then remain synchronized for many days (or months) after that thanks to low-drift crystal oscillators. And even without GPS, NTP exists.

1. Pragmatical Bustard says:

Dear Hair Brain,

Are you suggesting that every location should synchronize its time to local noon, having in effect an unlimited number or time zones? Or that should be only one, and here in Colorado I should be okay with the sun rising around midnight?

2. Finn says:

There’s certainly a discussion to be had there, but I don’t think clock synchronization, precision, or accuracy, is the issue. It’s more-so the long held social conventions where we associate certain times of day with specific activities. While dealing with different time zones can be inconvenient in some regard, it’s actually quite convenient in others. My job requires me to travel to, and interact across, multiple time zones. If we had one world wide clock it would be more difficult to know if, say, 3pm is an appropriate time to contact someone who’s based in another country. With time zones I know that if it’s 1am in a given place, that’s not a good time to call.

1. Lucas says:

And if it where all UTC, but X has their working day 10:15-18:30, Y from 12:00-20:30 and Z from 05:00-14:00 because they are all in different countries, it would be really easy to see that a teleconference between the three would need to be somewhere around 13:00.

Instead of timezones, you just have to keep track of what hours someone is available. And then that might be something as 23:00-06:00 but at least it would be in the same timezone as you, so no conversions and no possible errors in picking the correct timezone to convert too.

X is working a night shift by the way, because it is night at their location at those times.

Yeah, it has its pros and cons…

But no-one will complain anymore if you eat your lunch at 16:00. It might be noon at you location. (instead of ‘not gotten around to eating lunch yet’)

3. spaceminions says:

The whole point of having time zones is to not have to synchronize with distant places which have a different daily cycle because of their longitude, while limiting the complexity of communicating times. Being late to a meeting because you live to the west and were using your local time is bad enough, but time zones are a quick shortcut to let you still use actual numbers to describe a time of day without having to give an in-depth explanation of your latitude and longitude, the season, and the schedule you’re currently operating under, where the offset that ends up occuring could be any number of milliseconds and no-one knows the answer without a computer now. I get that some people operate under absolute time, but after a certain point I’m just going back to clanging bells in a tower for sunrise noon and sunset. :P

1. spaceminions says:

And yeah, the thing zerterer said, https://qntm.org/abolish can make it more confusing. Since days no longer mean what they used to mean, there will be more of having to say two days instead of one or having to figure out by coordinates, culture, and knowledge of whether a person has kids, what times they are likely awake. Which segments of time are weekdays and which are weekends? Who knows.

2. Daniel says:

“Arizona doesn’t change to daylight savings time. Oh, except for the Navajo Nation in Arizona, which does! ”

Except for the Hopi reservation, completely inside of the Navajo Nation, which *doesn’t* observe DST. And then except for a patch of the Navajo Nation completely inside the Hopi reservation that *does* observe DST. Makes for a fun map!

https://en.m.wikipedia.org/wiki/Time_in_Arizona

1. EmptyJay says:

You can walk in a straight line and change your clocks 6 times.

3. Finn says:

There’s certainly a discussion to be had there, but I don’t think clock synchronization, precision, or accuracy, is the issue. It’s more-so the long held social conventions where we associate certain times of day with specific activities. While dealing with different time zones can be inconvenient in some regard, it’s actually quite convenient in others. My job requires me to travel to, and interact across, multiple time zones. If we had one world wide clock it would be more difficult to know if, say, 3pm is an appropriate time to contact someone who’s based in another country. With time zones I know that if it’s 1am in a given place, that’s not a good time to call.

1. Ostracus says:

Property of nature and geography.

1. phuzz says:

It’s cultural/social/political as well. For example, daylight savings time comes from neither nature nor geography.

4. A says:

Fun fact, the 5.5 hrs difference between India and GMT means if you have an analog watch set to GMT you can easily tell the time in India by reading it upside down. This wasn’t an accident

1. Paul says:

Man, I can’t think of anything more colonial than that. Cripes.

2. abjq says:

We should have kept that trick for Australia!

1. I do timezones for a living (I work on a major calendering product you’ve probably heard of). And every time, _every_ time, I have to debug some godawful problem caused by timezones being wretched I watch that video and end up posting it to the internal chat.

Also worth reading are the comments at the top of https://github.com/pavkam/tzdb/blob/master/tz_database_latest/europe (and its sibling files). It goes into the history of timezones, and why they’re all wretched. Fun fact: Dublin’s time zone used to be -00:25:21. Yes, 21 seconds.

1. RonW says:

I recall reading, years ago, that Oxford’s time zone is/was -00:05:02

1. J_B says:

Because it is an offset from UTC, not GMT.

UTC is a universally coordinated time, which is determined from atomic clocks spread over the world and has nothing to do with astronomical events. It signifies the worldwide average as time is relative and deviations happen all the time.

GMT is astronomical time and noon is defined as the time where the sun has the highest position in the sky. (Actually it is the average over a year as the earth is not rotating at a constant speed.)

Basically, this means there is only a rough correlation between UTC and GMT.

If you want to have the sun highest in the sky at noon and still use UTC time, you end up with these wreird deviations.

2. ziew says:

Wasn’t it just Dublin’s solar time? It’s not like someone thought “lol, you know what, let’s set our watches exactly 25 minutes and 21 seconds earlier than GMT”.

5. echodelta says:

I wish Indiana was also one of the holdouts from what we Hoosiers call Daniels Stupid Time. The gov’nr that pushed it on the state then ran Purdue. I glanced at the zenith sun today through black felt in the south facing window, about 1:00 post meridian. Now we will soon do the time warp dance again, solar noon at 2P!

6. The Kenosha Kid says:

India has one time zone, Russia has eleven, Iran is fifteen minutes off.

7. Eric says:

Fun fact: China has only 1 time zone. The whole country is set around Beijing IIRC. People living in the far west side always got up and had breakfast before sun rises, and would be going to bed before the sun sets.

8. Lennart says:

And look at Australia. They have 6 timezones. Only some of them have DST and one differs 15 min and some other differ 30 min.
I live, as I see it at the moment, in the easy part of the world where we have CET now and CEST in the summer.

9. Rage says:

As someone in a half-hour timezone, I die a little inside each time I encounter a product that only has full-hour timezones. I’m looking at you NanoCam..

Also EST in Australia is further complicated as some states in that zone observe DST while others don’t.

1. MmmDee says:

Paul Eggert not Egbert

10. Lord Nothing says:

i live in a state that would normally span 4 time zones, but for some reason we only use one. yet we still use daylight saving time, especially when there are parts of the state that can be 2 hours off of normal and can result in clocks being off 3 hours from what the sun is doing. and its all moot because we have endless darkness in winter and endless daylight in summer (i remember doing a lot of midnight snack runs in broad daylight). especially when agriculture is concerned (very little actual night during the growing season). circadian rhythms be damned.

11. Paul LeBlanc says:

Canadians are used to announcements about upcoming TV shows being in the form “Tuesday at 9, 9:30 in Newfoundland”.

12. Mina Hatake says:

i find it funny that the photo for the article is from what looks to be ubuntu 8.04 my first real distro

13. Canuckfire says:

This became a very real nightmare for me when I started to get interested in timekeeping…
Making a thing “tick accurately” was fun, but I have basically come to the point where I want to connect it to the human-readable display part and what I *want* to do is have a central clock deal with the conversion to local time, but it is apparently sacrilege to have something like NTP actually make any adjustments to the time it sends to the clients.
The answer I keep seeing everywhere is to make the client do the conversion to local time, but then I need to deal with timezones and DST independently on dozens of client devices instead of once at the reference.
I can squint and understand that logically makes sense, but just feels so wrong and incomprehensively inefficient in practice.

14. Gerhard says:

And there was a time (sic!) before these modern time zones were introduced. Before the 20th century, practically every city in Europe had its own odd (local) time. The introduction of the railway and telegraphy eventually made it necessary to align local times. Before that, there was simply no need for simultaneity since there was no way to communicate or travel in “real time.”

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