# What would an Earth with no timezones be like?

If we abolished the system of time zones, and everywhere followed UTC (or another arbitrary timezone), what would the possible advantages or disadvantages of this system be? If you can think of any specific activities that would be disrupted or enhanced due to this, please include those as examples!

• Using UTC the world over might get people to realizing that clock time is something that has only a passing relationship to the phenomenon of time itself. – Howard Miller Jan 11 '16 at 20:57
• I am in disagreement that this should be closed, as we have questions related to eternal day or night planets. A cultural, geological, or astronomical phenomenon of a shared timezone doesn't seem that far off from those other questions, and the answers aren't so broad since timekeeping is so coupled with business and networking and living beings' habits. I'm just throwing my two cents in. – The Anathema Jan 11 '16 at 21:39
• @2012rcampion that article is becoming less relevant every day. pretty soon, if i want to call my uncle, i could just ask siri if now is a good time to call him. in fact, even without timezones, the author would not have known when to call him without siri's help since his uncle was planning on sleeping until solar noon. – james turner Jan 11 '16 at 22:47
• Software development time would be cut in half. – djechlin Jan 12 '16 at 21:36

For their day to day lives, people in different parts of the world would associate different times with different activities. While to someone in Greenwich 11:30 am might mean "lunch time", to people in the Eastern US it might mean "time to wake up", to others it might be "dinner time", or "middle of the night".

No doubt if time zones were abolished tomorrow, people would find it very confusing for a while. But I presume that after, what, a few years?, people would get used to it, and the idea that the sun rises at around 3:00 pm seem natural and familiar.

Once people were used to it, the only time when it would make a difference would be when someone travels to other parts of the world or talks to someone from other parts of the world. Today, for most of us there is some confusion when talking to someone far away as to what time of day it is for them. When it's 10:00 am at my home, what time is it in Tokyo? Etc. If there were no time zones, then this question would be meaningless. If it's 10:00 am here, then OF COURSE it's 10:00 am everywhere in the world.

But you'd still have to think about where the other person is in their daily cycle. When I ask, "When it's 10:00 am here, what time is it in Tokyo?", my reason for asking is likely to be, "Is this a reasonable time to call, or will it be the middle of the night over there?" You'd still have that problem. Well, I'm assuming that when you say there are no time zones, you're not thinking that everyone works to the same schedule. That, say, if people in London begin the workday at 9:00 am, that this means that everyone begins the workday at 9:00 am, even people for whom that is the middle of the night.

So on the one hand it could be simpler. We don't have to ask what time zone a person is in, because there's only one. If someone says, "the meeting is at 2:00 pm" or "the train leaves at 2:00 pm" or whatever, we don't have to ask what time zone. It could eliminate a lot of potential confusion. For people who regularly work with schedules across multiple time zones -- like people at railroads and airlines -- this could avoid a lot of potential confusion. I'm a software developer, and every now and then I work with computers located in different time zones, and this can get confusing. Like if I check a computer and see that a certain file was last updated at 3:00, is that my local time, or the time where that computer is located? And where is that computer physically located? Sometimes I don't even know. Etc.

On the other hand, it could make it more difficult to talk about different daily schedules. With time zones, it's easy to say "when it's 9:00 am here it's 4:00 pm in Tokyo", and we now have a good idea how our daily schedule relates to theirs. But with no time zones, how would you express this idea? You'd have to say things like, "people there usually start work at 4:00 pm". Then if you want to know when they might have lunch or go home from work or go to bed, you'd have to count hours from start of the day for you, and add it to their start time, and so do a bunch of arithmetic in your head, which could get awkward.

So, I'd say pros and cons.

• All file systems should record the 'last updated' time in UTC. Some don't. I feel your pain. – Joe Bloggs Jan 11 '16 at 17:25
• Well you wouldn't use PM - that's already a relative measurement from local noon..."post meridian". If everyone used Zulu (GMT time) you just would need to know when noon was in Zulu at any location you are interested in. For instance, the Central Time Zone is +6 hours from Zulu. – Oldcat Jan 11 '16 at 17:39
• @Oldcat CST is UTC-6, not UTC+6 – costrom Jan 11 '16 at 21:40
• I would like to point out that schedules vary across countries, and sometimes even firms/individuals. As a French, I'm always surprised when watching American movies because they get up much earlier than I do and, on the other hand, eat dinner much sooner, with regard to their local time. – Matthieu M. Jan 12 '16 at 7:25
• @molot Trains don't cross time zones as quickly, but they still cross them regularly, and it only takes a train crossing a time zone once in it's travel to create a potential for confusion. And airlines certainly do discuss schedules in local time: visit any airline website or look at the arrival and departure board at the airport and to the best of my knowledge they are always expressed in local time. Airline agents talking to customers give times in local time. Maybe in purely internal conversations they use UTC, so there might be LESS change, but not NONE. – Jay Jan 12 '16 at 19:09

Look at China. China already abolished timezones (within the boundaries of the country).

The result would be that "9am" is not breakfast for everyone, instead "9am" would be an objective point in the day at which different people do different things.

So for you, living in California (for example) with the UTC model, "9am" is the middle of the night. You won't be up until 3pm, leaving for work by 4pm, getting there at 5pm, and working until 2am. All during the daylight hours.

Whereas for someone living in the UK "9am" would approximately be when they get to work.

This would cause more headaches than it solves, as your UK compatriot would send you an email asking for an 11am conference call and you're reply would be "That's the middle of the night!" Oh, he replies, what's the time difference? and you have no reply because the concept of time zones is dead.

• I never realized that getting rid of time zones would actually make it harder to plan meetings across time zones. I always thought that everyone using a common time would be futuristic, but now I'm just thinking about people trying to figure out what time it is by comparing the position of the sun, or how long it's been since breakfast, and it sounds like a terrible idea. In other words, +1. – DaaaahWhoosh Jan 11 '16 at 16:47
• It barely holds together in China, and that's only got five (standard) timezones to deal with. And the only reason it's kept is because the government is a borderline dictatorship. Whatever the Communist party wants the Communist party gets and someone decided that having everyone use the same timezone would be the best of ideas. And so the common folk suffer with pre-dawn 9ams. – Draco18s Jan 11 '16 at 16:53
• @Kromey To be fair, that far north the clock stops making sense (with the sun's actions) anyway! Only a little farther and the sun won't set at all! – Draco18s Jan 11 '16 at 18:53
• The upshot of my comment (ran out of space) being that things like "pre-dawn 9ams" really aren't that big a deal. I get to work at 8am, whether the sun's up or not, and I work until 5pm, whether the sun's set already or not. What is a big deal, though, is that when I try to coordinate with European contacts, I'm getting to work at 8am my time, which is 5pm (give or take depending on country) over there -- and that would only be worse if we're both technically on the same time, but I'm going to work at 5pm whiel they're clocking off at 5pm. – Kromey Jan 11 '16 at 18:53
• I think your conversation might go more like, "conference call at 11am?" "I am asleep at that time!" "What times are you available?" "I'm available [these times] - (which the caller compares with the times he is available - rather than converting to his timezone and then comparing). The hindrance occurs when trying to plan a meeting time under some assumptions - when you don't have contact with the other person but you know their location and thus what time they should be available - such as "he should be in the office between 8 and 5 his time which is [x] my time" – DoubleDouble Jan 11 '16 at 20:46

For one serious difficulty, look to people working night shifts, where the date switches over in the middle of the night.

Knowing what time you go to work is easy. The difficulty is with what date you are on. We are all used to the date changing at midnight, when most people are asleep. The next day, you date any papers with a new date, that you keep for the entire waking cycle.

In the new system you have to change the date at midnight UTC. Depending on where you are, this could be a couple hours before the solar clock reaches noon. It's very unintuitive, and gives people in this situation something extra to think about. As a result, it would cause problems, even among people used to it. It is already somewhat difficult to keep track of a current date. Changing it midway through the waking cycle just adds confusion.

From personal experience, I worked an overnight shift where we had to record the time of certain events, ending at dawn. When the day switched over at midnight, we technically switched dates. In practice, when we wrote 1AM, July 18th, it was 1AM of the night that started on July 18th. We had tried changing dates at midnight, but even using a computer that told up the actual date, people made frequent mistakes, often continuing to use the previous date all night long. It meant a lot of going through the paperwork to correct dates. Not changing dates midway through the worknight saved a lot of trouble.

• I love this, totally makes sense, had not occurred to me at all. – Spike0xff Jan 12 '16 at 14:52
• TV channels already sometimes have this problem, where they have to describe something happening at "1am on Saturday night / Sunday morning", because they feel "1am Sunday" wouldn't make sufficient sense to somebody who thinks as "Saturday" as occurring from when they get up Saturday morning to whenever they go to bed the following night. The danger with writing "1AM July 18th" to mean 2016-07-19T01:00, is that if your writing is ever divorced from its original context then all the errors hit anyway. But as you say that risk might still be better than definitely making errors all the time. – Steve Jessop Jan 12 '16 at 16:03
• Generally, I see people just write "shift started 18 July, 8 PM", "first entry, 9 PM", "second entry, 1 AM", etc. In some of my hospital records, there's an entry before the first log after midnight expressing the new date, then everything after is just the time. – MichaelS Jan 13 '16 at 9:28

Why abolish just the timezones? Timezones are merely a tool to more-or-less successfully force "day", a time unit based on Earth rotation, into conformance with local solar time.

I'd say - get rid of this geocentric nonsense alltogether!

Welcome kiloseconds and ultimately metric society! Think of Unix time - just a number, ticking arbitrarily and not being tied to timezones, planet rotation, moon orbits nor planetary orbits.

Plot twist: it works perfectly even between different planets, moons and space stations. For a distributed society it would be a big advantage.

Firstly, if timezones are abolished and UTC time used everywhere then we would also need to simultaneously abolish the use of 12-hour time, i.e. AM and PM, as this is now redundant and confusing terminology and use 24-hour time instead.

Secondly we currently use time as an indicator of the local day/night cycle as well as measuring the passage of time. For example if someone tells you it's 6 AM where they are, then you know that roughly speaking this will be around dawn / early morning. However using only UTC if someone tells you it's 06:00 then it is 06:00 for both of you, but is it dawn or dusk where they are? I think people would still need a secondary system for indicating relative time of day in their locality.

Working in IT I can say that timezones are a major headache in software applications, as they are defined by legislation and can be changed arbitrarily. This is one reason UTC is popular in software applications.

• I've heard of the nightmare of working in the scheduling department at Hoover Dam. Some of the workers are from California (Pacific), some are from Nevada (Mountain). California goes on daylight savings, Nevada doesn't... And to make it even more fun, IIRC, the Native American workers who live on reservations in the area do follow daylight savings. So... yea. – user487 Jan 12 '16 at 16:35
• @MichaelT I don't think that is correct. Nevada does have daylight savings and is on Pacific time. Maybe you are thinking of Arizona? Arizona does not have daylight savings and uses Mountain time. Note that Hoover Dam is on the border between Nevada and Arizona, more than an hour from the California border by car. This may change next year, as Nevada has asked to switch to PDT year round, effectively the same as Arizona (MST year round). – Brythan Jan 13 '16 at 5:22
• And the stupid date line could be straightened out at last. I write software to calculate flight paths of objects, and make sure they don't intersect scheduled flight corridors and so. That thing causes me so much trouble. – RedSonja Jan 13 '16 at 12:25
• @Brythan You are right, I was thinking of the wrong pair of states... it is Nevada and Arizona. – user487 Jan 15 '16 at 2:47

There are two ways this could go. Most of the answers assume that all timezones but one (generally GMT/UTC) are abolished and all our clocks are set to the same time. People will just have to get used to having breakfast at midnight, or going to bed at 2pm, if they are unlucky to live in a place where that is the custom.

But the other way would be to return to astronomically correct time, wherever you are. In normal rural life, ... no problem. If you have to communicate with another location, you need to calculate the correct time wherever they are - for example, in Bristol you'd better turn the TV on at 5:40pm to catch the 6 o'clock News from London.

Then there's the discrepancy between solar time and mean time, defined by the Equation of Time. Your clocks (which drift) run on Mean Time, but your sundial (which is accurate) reports solar time, and they can be up to 15 minutes apart in either direction at different times of the year, so even in one location you need to convert from one to the other with a calendar and a pocket calculator...

What day it is - now that's a whole different problem :-)

I suspect it would be very like the situation with months that we have now. In the UK, January is considered likely to be cold, possibly snowy, and generally a really bad time to have a barbecue. In New Zealand, January is early summer, warm, sunny, and absolutely ideal for barbecues.

In the same way, if everyone worked on UTC, the UK would be pretty much unaffected (work starts at 9am, and goes on until 5pm, or something like that). Australia would effectively flip AM and PM, so work starts at 9pm, and goes on until 5am - someone starting work at 9am there is on a night shift, and someone working at 5pm is gearing up for the breakfast rush. It would take a little getting used to, but people have adjusted to a lot more (Christmas on the beach!)

The more interesting changes are all the places in between. In the west coast of the USA, 1pm would be the starting time (roughly), but there would probably be some determination of what a reasonable work time was, either from the top down (government says a reasonable working day is 8 hours between X and Y), or from businesses choosing when seems reasonable, and everyone else fitting with them.

In an ideal world, businesses might take the switch as an opportunity to desync with the local rush hours - there is a lot of cultural inertia behind the idea of "9 to 5", but if local businesses worked a wider range of start times, then peak traffic could be spread across a wider timespan. That would reduce jams, but would rely on businesses being willing to take a risk on being out of sync with others in the area. It's fine if you start a bit before your main customers do, unless they often have issues last thing in the day!

In our world, though, I suspect that businesses would stick to exactly the hours they work now, just with different names. Working 9-5 would happen in the UK and Australia, but working 1 til 9 is still a way to make a livin'

on the plus side: almost everything that uses timezones would be simpler. specifically, when traveling or communicating across timezones. this includes reading things written in another timezone. notable examples include:

1. booking airfare
2. scheduling distant meetings (either via telecommunications or travel)
3. setting clocks (since daylight savings time wouldn't make sense and clocks could come pre-set)
4. knowing when emails, text messages and internet messages were sent
5. writing software of almost any kind (software frequently uses timestamps)

on the down side:

1. it might be harder to predict when stores open and close. e.g. at some longitude 7-eleven stores would switch from opening at 7am to 6am. this would probably follow political boundaries like state lines much like timezones, but be determined on a chain-by-chain basis rather than state-by-state basis. of course, now that we have google maps, that would be much less of an issue.
2. also, once you have gotten past your jet-lag, it might be slightly more confusing to make dinner reservations because you might not know off hand when you will be hungry, that is however a very minor pause rather than a big issue.
3. lastly, determining when businesses would be open (and people are awake) in distant places would be different. instead of checking the timezone offset and assuming 9-5, there would probably be an official state "business hours". this system would in fact be more effective since not all regions use 9-5. e.g. "temperate" latitudes tend to be closer to 8-4 to optimize daylight during commute, while tropical climates tend towards 10-6 to minimize commuting during the afternoon heat. some regions have longer split work-days with long lunch/rest breaks in the middle, while other regions have shorter workdays due to labor laws.

side note regarding decimal time:

on the whole, humans are unlikely to eliminate timezones until we switch to a decimal timekeeping system. both "french republican calendar" and its modern revival "internet beats" use timezone-free decimal clocks whose date flip happens in sync with traditional france time (gmt+1:00). unfortunately, decimal time seems unlikely to gain broader usage until we have a non-trivial polar or non-earth population. even then, there are likely to be several decades during which both systems are used (e.g. today we use 12hr and 24hr clocks). also, emerging technologies make the transition easier every year. e.g. soon you will be able to ask siri "when would be a good time to call my uncle in japan?", and she will consider average local business hours as well as when your uncle specifically tends to have online activity. meanwhile, if you would like to start using a decimal clock, there are several mobile apps for it. unfortunately, most of those clocks will shift around as you move between timezones. oddly enough, the only decimal clock app i have found that allows you to lock the timezone is the startrek clock app, since one version of "stardate" was essentially the french revolutionary system.

• Business hours and bank hours get even more awkward than you've depicted. If you deposit at 11:50 PM and withdraw at 12:10 AM, has it been in the bank a day? Well, that's absurd since banks aren't open those hours. But if its UTC based, in Hawaii one could do something like that. – user487 Jan 12 '16 at 16:37
• it doesn't seem awkward to me. the cut-off for deposits at my bank is 5pm eastern. so if i got to the bank at 4:49pm eastern to make a deposit, then in 2 minutes and make a withdrawal, the bank will consider the funds "in the account" for 1 day when calculating my average daily balance. without timezones, this cut-off would probably be at the start of the day for every bank in the world, rather than varying from 3pm to midnight local time depending on the bank. – james turner Jan 12 '16 at 17:16
• You mean 4:59 pm unless where you live you have 50 minute hours! – CJ Dennis Jan 13 '16 at 7:34
• lol. indeed i do :) hard to remember how many units you need to rollup when you aren't using decimal time ;) – james turner Jan 13 '16 at 14:09

Here are two things that nobody seems to have touched on: the definition of days of the week; and related resistance to the change.

How do you define a "day"? To put it another way, what is "Monday"? Is it a 24-hour period starting at 00:00 and ending at 24:00? If that's so, then Russ from Auckland never "works Mondays". He starts work at 21:00 Sunday and knocks off at 05:00 Monday. Meanwhile, Mona from San Francisco starts her working week at 17:00 Monday and doesn't finish until 01:00 Tuesday.

The most plausible alternative is a "mixed" system, whereby day names are associated with local solar days, regardless of the time. This is going to be annoying and inconsistent, but it does have a certain advantage. Let's say you wake up "Monday morning" in Auckland, in the early morning light at 19:00. When 24:00, noon, rolls around, what comes next?

• If Monday means the local solar day, then "Monday morning" is followed by "Monday afternoon".
• If Monday means 00:00 to 24:00, then "Monday morning" in Auckland is followed by "Tuesday afternoon"!

And then there's a the second point: cultural, and especially religious, resistance to this change. (Don't underestimate this; it's arguably what sank metric time in Revolutionary France.)

Let's say Russ goes to church on Sunday morning, at 22:00. He's already been observing the Sabbath for 22 hours, and his friends who go to the evening service went before him, not after like they did in the old days. It's also awkward when the service is followed by a church lunch that runs into Monday afternoon. But that's not what really bothers his conscience. He knows that in his grandparents' day, they observed the Sabbath from (what he now calls) 12:00 Saturday until 12:00 Sunday. Is his church really keeping the Sabbath? It's the Seventh-Day Adventists all over again! And what do they do with Good "Friday"?

Judaism may have an easier time of it, because the Shabbat is already technically observed according to a conversion of systems: it just so happens that the last day of the Hebrew week starts on what we presently call "Friday". So Mona, who's Jewish, observes Shabbat from 02:00 (sunset) on Saturday until 02:00 on Sunday.

Cory Doctorow's free (released under a Creative Commons license) book "Eastern Standard Tribe" describes a related situation.

In the story, people belong to "tribes" that share a reference timezone, e.g. the Eastern Standard Tribe, whose reference time is according to the EST timezone. All people who belong to a tribe try to maintain a circadian wake-sleep cycle appropriate for that timezone, regardless of where they physically reside.

Advantages of this culture include making the communication among members of a tribe straightforward, and eliminating the jet lag and related hassle when physically travelling. But, as might be expected, the situation creates animosity between the different tribes, each of them, in a sense, living in a parallel world. Also, in each latitude, the tribe whose timezone best fits the human body's natural circadian rhythm is better adapted, and has a "home advantage".

ADDENDUM (trying to better relate to the question): I don't remember the history of the world in the story (and I'm not sure it was explored), but I assume that people tried to abolish time zones, like the OP suggested, but failed to do so. Instead, there arose a competing set of standards tribes, where each tribe enjoys the advantages of the abolition within itself. One might conjecture that later in the history of that world, a single tribe would emerge victorious and impose its timezone upon the rest, thus leading to the world in the question.

• Welcome to the site yoni, while interesting this is more commentary than an answer to the question. Please take a moment to edit and address the question posed in the OP. – James Jan 13 '16 at 15:18
• @James, did what I could to elaborate the answer, and better fit to the question. – yoniLavi Jan 13 '16 at 15:35

The only way you can get rid of time zones without problems is if you could remove the dependence on the sun.

This is impractical on Earth.

So other options:
If we were ever to try to colonize Titan, the sun would just be a bright star in the sky. It's also really really cold there.
It does have an atmosphere, which makes it attractive to colonizing though. So you make it possible by orbiting several fusion reactors around the planet. Because they are ringing the planet, you could light the entire surface at once, and turn the fusion up or down to simulate day/night. Maybe refuel the reactors at night.
Since the entire planet is lit evenly, you could just have a single time that everyone uses.

A multi generation ship would have to be pretty big.
In a space ship, there would be no day or night, just ship time.
Not quite a world, but if it was big enough it could feel like it...

• Titan still has day and night. Although the Sun is roughly 100 times dimmer when seen from Titan, it is still far brighter than any other star in the sky. Compare the day and night sides in this NASA image. (Do note though that the day/night cycle is something like 380 hours long.) – 2012rcampion Jan 11 '16 at 22:39

There was already a world without timezones for a longer time than with. Before the railroad occured, every place in the civilized world defined 12:00 as the time where the sun was on it highest point. But as railway transportation took place, it was nesseccary to have one national time just to be able to create schedules for it. So the real deal would be not to change for UTC, instead it would be right to change to real local time! Today this would be no problem at all, because we have computers and GPS which could calculate this real time with ease. In that case, no time zones would be needed any longer, just UTC and the GPS-Coordinates to calculate real local time. Of course this would also make summer-/wintertime obsolete!

• umm, ok? doesn't really make much sense to me... – bowlturner Jan 12 '16 at 20:30

It also makes "day" milestones more difficult to determine. Especially if you live somewhere like I live, where the date would generally change as I'm driving home from work or eating dinner.

• When do you celebrate your birthday?
• What date did you get married on—does this depend on whether you got married in the afternoon or evening? What about weddings that last for several hours between the service and the reception?
• Quite relevant to legal systems, when are you eligible to vote? Voting time will be sunrise to sunset (say), but what if the day changes while the sun is in the sky? Do you have to turn 18 (in the US) on the first date, or can you turn 18 (in the US) on the second date during daylight?
• Does Midnight Mass (for Western Christians who celebrate it) happen at 23:59 UTC, or does it happen at local midnight? If at local midnight, is that local midnight of the 24th or the 25th?

Either way you pick these, they end up a bit odd, and totally contrary to how humans operate and think.

• I don't see any of those being weird. Right now, my birthday is celebrated relative to wherever I'm living, not relative to 2 AM in Texas where I was born. In the new system, there would be one date everywhere, and I would celebrate on that date. Same with marriages, holidays, voting, etc. Nothing has changed. Things like New Year's Eve would be simplified since we'd celebrate it once worldwide instead of watching 24 countdown clocks until we pass out. Things like Midnight Mass would depend on local custom. Just like celebrating at 2400 instead of solar midnight is a custom. – MichaelS Jan 13 '16 at 9:40

The real problem is not so much timezones, but rather keeping accurate clocks and synchronizing them across long distances. Whatever standard you pick doesn't really matter so much - it's having the standard that makes by far the largest difference. If it's UTC 3:32 in New Yord and it's UTC 3:32 in Tokyo that's fine - at least you still know what time it is, and it's reasonably convenient.

Before time zones, accurate clocks were expensive (and not that accurate). Large public clocks formed a local reference for people in a given town or city and those clocks were set by the sun at local noon (often using something like a sextant, same as a sailor would at sea). This meant that 2:10PM in Chicago was maybe 2:34PM in Cincinati, 1:53PM in Kansas City, 2:42PM in Pittsburg and 3:01PM in Philadelphia, etc. The chaos came from the train system, where you had to leave one city and catch a train in another, scheduled to leave at a specific local time that was different from the local time you were leaving - possibly moving past several other cities in the meantime.

Rather than a single time reference, you would have needed five or six or more pocketwatches and a spreadsheet of calculations to figure out which train you needed to take to make another at a given time in a given city to be at your destination at the local time you needed to be there. That was the nightmare before timezones. Standardizing time and synchronizing everyone's clocks, doubly so with very accurate clocks, is what makes for functional interaction over long distances. The particular details of the standard don't really matter - people adjust and get used to whatever that standard is, whether it is their local timezone time, UTC, or Stardate 14352.6. What matters is that everyone everywhere agrees on what time it is and can communicate that to each other in a clear and easily understandable way.

In a way, a single standard is actually clearer and easier because you need one fewer piece of information - the timezone. Militaries (and airlines), for example, almost always work on UTC (aka : Zulu) time because their operations are frequently crossing timezones and a single time standard ensures that everyone is absolutely clear on where and when things are happening.

First, we'd still have time zones after a fashion. We'd all have an "offset" to describe how much my time differs from UTC, and how much yours does, then I would subtract my offset from yours and add the offset to my clock to figure out what effective time it is where you live.

For example, I'm in UTC-7 and you're in UTC-5. Yours-Mine = -5-(-7) = -5+7 = 2. So if it's 03:35 here, it's like 05:35 there. Which is what we do already to determine the actual time. So that wouldn't change.

Second, we'd either have a secondary set of local clocks that add our own offset to UTC -- oh wait, that's what we're doing now. Or we wouldn't.

Let's say people were somehow convinced to always use UTC. What would happen?

• People in different parts of the world would think about "9 AM" in different ways. People in Europe/Africa would think of it as morning. People in the Americas would think of it as afternoon. People in Asia would think of it as night.
• People who work a shift where the date changes would have to get used to changing the date in the middle of their shift. Because most people work daylight shifts, this would affect a larger percentage of the population that it currently does.

That's really about it. "9 AM" already means something very different to me than my mom. For me, it's the middle of my sleep cycle. For my mom, she's already eaten breakfast and fed the animals on the farm. Between noon and 7 PM we're both awake, although I'm on the just-woke-up part of my day, and she's on the about-to-go-to-bed part of hers. So we've already fixed that problem: just ask your friends what time they're normally available and on what days, then figure out what part of that window coincides with your availability.

And lots of people already have to deal with date changes midway through their shift. For most people, it's not relevant. If I sign a check or datestamp my logbook with yesterday's date, nobody cares. For people whose jobs requires the correct date, software normally does it for them. Otherwise, they can look at the time. If it says 00:XX or later, it's tomorrow. If it says 23:XX or earlier, it's yesterday. At the beginning of your shift you'd write both dates for easy reference.

You don't even have to convert to a 24-hour format. The AM/PM system is completely unambiguous. There are exactly 24 distinct hours per day. The date changes on the PM -> AM change. It's even in alphabetical order for you.

I would be happy if the AM/PM system didn't follow 11 AM with 12 PM. It needs to be 11 AM -> 0 PM or 11 AM -> 12 AM -> 1 PM (noon), but that's true regardless of timezones or a lack thereof.

Time zones are not man made. They have always existed. The majority of people in the world are awake during daylight hours and asleep when the sun is down. It is a natural and expected phenomenon. Ancient sundials exist all over the world including the US and Canada supporting the notion that human beings are highly organized and extremely intelligent creatures.

• Sure time zones are man made, they have not always existed. Local times (as read by a sundial) are not man-made; but for anything where you need synchronised time between remote locations somehow (first necessary with the advent of railways) it's totally impractical if you always need to know the exact longitude difference. That's why sundials got out of use. Time zones already forego local time; as already said, in vast time zones like China this is very notable. Zones are in many ways a poor compromise between proper local time and proper global time. – leftaroundabout Jan 12 '16 at 10:23
• to the irritation of engineers and computer scientists, sometimes the best solutions are messy compromises ;-) – Spike0xff Jan 12 '16 at 14:45
• @leftaroundabout: local times are time zones. There just used to be a different time zone for every village, then we condensed it to 30 or 50 or something. And to be fair, it's really not hard to do longitude math. I'm at 111° W = -111°. Let's say my friend is at 83° E = +83°. 83°-(-111°)=+194°. There are 360° in a circle. +194°/360° = +0.539. With 24 hours in a day, thats +0.539*24 hours = +12.9 hours. 0.9 hours is 0.9*60 minutes = 54 minutes if we care. The equation simplifies to (destLong-originLong)/15. Or divide by 3 to an extra digit or two then again by 5 if division by 15 seems hard. – MichaelS Jan 13 '16 at 11:47