It sounds like such a simple question. You glance at your phone, you read the digits, you get on with your day. But behind those digits is a tower of compromises, conventions, and politics that has taken humanity thousands of years to build – and it’s wobblier than you’d think.
Sticks in the ground
Timekeeping started the way most useful things start: someone needed to solve a practical problem.
If you’re growing crops, you need to know when to plant. If you’re a priest, you need to know when to perform a ritual. If you’re a trader, you need to know when the market opens. The sun moves across the sky in a predictable arc, so you stick a pole in the ground and watch its shadow. Congratulations: you’ve invented the sundial, and you’re roughly in agreement with everyone else in your village about when noon is.
The Egyptians were dividing daylight into twelve parts around 1500 BCE. The Babylonians gave us base-60 counting, which is why we’re stuck with 60 minutes in an hour and 60 seconds in a minute – a convention so old that nobody alive chose it, yet nobody can change it.
But sundials only work when the sun shines. So people got creative.
Water clocks – clepsydrae, from the Greek kleptein (to steal) and hydor (water) – measured time by the regulated flow of water from one vessel to another. The ancient Egyptians used them by at least 1500 BCE, and versions appeared independently in China, Greece, and Rome. Some were remarkably sophisticated. The Chinese polymath Su Song built a water-powered astronomical clock tower in 1088 that stood ten metres tall and featured an escapement mechanism – a device for converting continuous water flow into regular, counted intervals – centuries before European clockmakers would independently develop the same idea (Joseph Needham, Science and Civilisation in China, Vol. 4, Part 2).
Candles as clocks were simpler but clever. You’d mark a candle at regular intervals and burn it down, reading the time from how much remained. King Alfred the Great is traditionally credited with using graduated candles to regulate his daily routine in the 9th century, though the story is likely embroidered. The truly ingenious trick was the candle alarm clock: push tacks or small nails into the wax at a specific height. When the candle burns down to that point, the tacks fall onto a metal plate below with a clatter. You’ve just been woken up by a piece of wax and some hardware.
Hourglasses – sandglasses, really – became widespread from the 14th century. Cheap, portable, and indifferent to weather. Ships used them to mark watches. Churches used them to time sermons (some congregations reportedly installed them facing the preacher, as a gentle hint). Kitchens used them, and still do. An hourglass doesn’t tell you what time it is – it tells you how much time has passed, which is often what you actually need.
Village clocks mattered more than any personal timepiece for most of human history. Before wristwatches, before pocket watches, the church or town clock was the time. Its bell rang the hours, and entire communities synchronised their days to one mechanism in one tower. If that clock drifted, everyone drifted with it, and nobody noticed because there was nothing else to compare it to. Time was communal, and it was local.
For most of human history, this was fine. Noon was when the sun was highest where you stood, and what noon meant three towns over was someone else’s problem.
Clockwork
The mechanical clock changed everything slowly, then all at once.
Weight-driven clocks with verge escapements appeared in European church towers in the late 13th and early 14th centuries. They were large, expensive, and not particularly accurate – drifting by perhaps fifteen minutes per day. But they worked at night, they worked in rain, and they kept the whole town on the same schedule.
Then Galileo noticed something. In 1583, as the story goes, he watched a lamp swinging in the Cathedral of Pisa and timed it against his pulse. Every swing took the same amount of time, regardless of how far the lamp swung. He’d observed the isochronism of the pendulum – though he never built a pendulum clock himself.
Christiaan Huygens did. In 1656, the Dutch mathematician and physicist built the first working pendulum clock, and the leap in accuracy was extraordinary: from roughly fifteen minutes of drift per day to about fifteen seconds per day. That’s an improvement of roughly sixty-fold. Huygens patented the design the following year and published the theory in Horologium Oscillatorium (1673), one of the great works of 17th-century physics.
From there, the history of timekeeping is a history of progressive miniaturisation. Tower clocks became mantel clocks. Mantel clocks became pocket watches as mainsprings replaced hanging weights and balance wheels replaced pendulums (a pendulum, after all, needs gravity and a stable surface – useless in a trouser pocket). Pocket watches became wristwatches. Each step required new engineering: smaller parts, better lubricants, more precise machining. The craft of watchmaking drove precision manufacturing for centuries before the Industrial Revolution made it commonplace.
Calendars
While clockmakers worried about hours and minutes, astronomers worried about days and years – and the calendar systems they built are a beautiful mess.
The fundamental problem is that the Earth’s orbital period (365.2422 days) doesn’t divide neatly into lunar months (29.5306 days), and neither divides neatly into days. Every calendar is a compromise.
The Gregorian calendar – the one most of the world uses for civil purposes – counts years from an epoch chosen by a 6th-century monk named Dionysius Exiguus, who was trying to calculate the birth year of Jesus and got it wrong by several years – but we’re stuck with his guess now and uses a solar year: 365 days, with a leap day every four years, except every hundred years, except every four hundred years. It was introduced by Pope Gregory XIII in 1582 to fix the drift of the Julian calendar, which had been gaining roughly three days every four hundred years against the seasons. The Julian calendar, introduced by Julius Caesar in 46 BCE on the advice of the Alexandrian astronomer Sosigenes, had been good enough for over a millennium. But by the 16th century, the vernal equinox had drifted from March 21 to March 11, and Easter was wandering uncomfortably.
The switchover was not smooth. Catholic countries adopted the Gregorian calendar in 1582. Britain and its colonies didn’t switch until 1752 – losing eleven days overnight when September 2 was followed by September 14. (The popular story of “Give us our eleven days!” riots is probably apocryphal, but the confusion was real.) Russia didn’t switch until 1918, after the revolution. Greece held out until 1923. This means that for centuries, different countries were on different dates at the same time. The October Revolution? It happened on 25 October 1917 by the Julian calendar Russia was using – which was 7 November 1917 by the Gregorian calendar the rest of Europe had already adopted. An October revolution that happened in November.
The Islamic calendar is purely lunar: twelve months of 29 or 30 days, totalling 354 or 355 days per year. It doesn’t try to track the solar year at all, so its months rotate through the seasons over a roughly 33-year cycle. Ramadan falls in summer some decades and winter in others.
The Hebrew calendar is lunisolar – it uses lunar months but adds an extra leap month seven times in every nineteen-year cycle to stay roughly aligned with the solar year and the seasons. The Chinese calendar works similarly, with animal years cycling through a twelve-year period.
The Ethiopian calendar is seven to eight years behind the Gregorian – Ethiopia is currently in the 2010s by its own reckoning, and celebrates New Year on September 11 (or September 12 in a leap year).
Not all calendars count up. Roman calendars counted backwards from three fixed points in each month – the Kalends (first day), the Nones (fifth or seventh day), and the Ides (thirteenth or fifteenth day). Julius Caesar was assassinated on the Ides of March – March 15, 44 BCE. And the Maya Long Count, which tracks elapsed days from a mythological creation date, is arguably a countdown to the end of a great cycle – the one that generated all that apocalypse hysteria around December 2012.
The problem that made time matter
In the 18th century, ships were sinking because sailors couldn’t figure out where they were. Not north-south – that part was easy. You measure the angle of the sun or the North Star above the horizon and you’ve got your latitude, your position north or south of the equator. Sailors had been doing this reliably for centuries.
The deadly question was east-west. Longitude – your position east or west of a reference point – was a different beast entirely, because longitude is fundamentally a time problem. The Earth rotates 360 degrees in 24 hours, which is 15 degrees per hour. If you know it’s noon where you are and you also know it’s currently 3:00 PM back at your reference point, you’re three hours west – 45 degrees of longitude. Simple arithmetic.
The catch: you need to know what time it is somewhere else. And in the 18th century, no clock could survive months at sea. Pendulum clocks were hopeless on a rocking ship. Without a reliable way to carry a reference time, sailors were guessing their longitude – and guessing wrong meant hitting rocks.
On 22 October 1707, a fleet of Royal Navy warships under Admiral Sir Cloudesley Shovell was returning to England through the Western Approaches. Fog. No sun for days. The navigators estimated they were safely west of the Isles of Scilly. They were wrong – they were further east than they thought, because they had no way to know their longitude precisely. Four ships struck the rocks off Scilly. The Association, Shovell’s flagship, went down in minutes. Nearly two thousand sailors drowned. It was one of the worst maritime disasters in British history, and the root cause was that nobody on board could answer the question “how far west are we?”
A clock keeping Greenwich time would have saved them. Here’s how: even in fog, a navigator can estimate local noon from the behaviour of the sea and the dimming of the light. If local noon happened when their Greenwich clock read 12:40 PM, that’s a 40-minute difference – which means they were 10 degrees west of Greenwich. The Isles of Scilly are at about 6.3 degrees west. Ten degrees would have put them safely past. But their dead reckoning said they were further west than they actually were. With a clock, a glance and some arithmetic would have shown them the truth: they were closer to Scilly than they thought, and they needed to change course. They didn’t have a clock. They hit the rocks.
The disaster was so shocking that Parliament offered a prize of £20,000 – millions in today’s money – for a practical solution. The Longitude Act of 1714 established the Board of Longitude, and the race was on.
John Harrison, a self-taught carpenter and clockmaker from Yorkshire, spent decades building a series of marine chronometers, each one a masterwork of engineering. His H4, completed in 1761, was a pocket-watch-sized device that lost only five seconds on an 81-day voyage to Jamaica. The Board of Longitude, staffed largely by astronomers who preferred a celestial solution, dragged their feet on paying him. Harrison eventually got his money, but he was 80 years old by the time the matter was fully settled. Dava Sobel’s Longitude (1995) tells the story beautifully.
It’s hard to overstate the impact. Accurate portable clocks didn’t just solve navigation – they made the modern world possible. Once you can coordinate time across distance, you can coordinate anything across distance.
Railways ruin everything (in a good way)
For a long time after Harrison, local time persisted on land. Bristol is about 2.5 degrees west of London, so noon in Bristol is roughly 10 minutes after noon in London. Nobody cared, because the fastest you could travel between them was by horse, and ten minutes didn’t matter.
Then came the railways.
If a train departs London at 8:00 AM London time and is due in Bristol at 10:00 AM, is that 10:00 AM London time or Bristol time? Now multiply this confusion by every station on every line. Timetables became dangerous nonsense.
The Great Western Railway adopted Greenwich Mean Time across its network in 1840. Other railways followed, and by 1880 GMT was the legal time for all of Great Britain. The rest of the world gradually fell into line, carving the globe into time zones at the International Meridian Conference in Washington DC in 1884.
Time zones and their discontents
Time zones are, fundamentally, a hack. They pretend that everyone within a wide strip of the Earth shares the same local time, which is obviously not true. And they’re political as much as they are geographical.
The zones are not neat strips. They follow national and regional borders, creating wild zigzags on any map. Spain is geographically in the same zone as the UK and Portugal but uses Central European Time (UTC+1) because Francisco Franco aligned Spain’s clocks with Nazi Germany in 1940, and nobody ever changed them back. The sun sets absurdly late in Madrid in summer. Western China is officially UTC+8 (Beijing Time) but the sun doesn’t rise until 10 AM in winter in Kashgar, near the border with Kyrgyzstan. The entire country uses a single time zone because Beijing says so.
Half-hour offsets. India uses UTC+5:30, a compromise between the needs of Mumbai in the west and Kolkata in the east. Iran is UTC+3:30. Afghanistan is UTC+4:30. Myanmar is UTC+6:30. Newfoundland, that gloriously stubborn Canadian province, is UTC-3:30. The Marquesas Islands in French Polynesia are UTC-9:30. Sri Lanka briefly switched from UTC+5:30 to UTC+6 in 1996, then switched back six months later.
Forty-five-minute offsets. Nepal uses UTC+5:45 – the only country in the world on a 45-minute offset from UTC. New Zealand’s Chatham Islands use UTC+12:45.
The fourteen-hour offset. Kiribati’s Line Islands are UTC+14, making them the first place on Earth to enter each new day. They didn’t earn this naturally – Kiribati changed the time zone of its eastern islands in 1995, skipping an entire day (December 30 simply didn’t happen for people in the Line Islands), so the whole country would be on the same side of the International Date Line. Before the change, Kiribati straddled the line, meaning it was simultaneously two different days on two different islands of the same country.
Zones offset more than you’d expect. France uses UTC+1 despite Brest being west of Greenwich. Western Argentina runs on UTC-3 even though its geography suggests UTC-5. Spain, as noted, uses UTC+1 instead of its geographically natural UTC+0.
And then there’s Eucla. On the Eyre Highway near the Western Australia-South Australia border, a handful of roadhouses and a telegraph station use UTC+8:45 – an unofficial timezone that splits the difference between Western Australia’s UTC+8 and South Australia’s UTC+9:30. It’s not recognised by any government. It’s not in any legislation. The locals just decided that neither neighbouring timezone made sense for them, so they invented their own. The IANA database doesn’t even have an entry for it – it falls under Australia/Eucla with a +8:45 offset, one of the most obscure timezone entries in the world. If you’re driving from Perth to Adelaide and you stop for petrol at Border Village, you’re in a timezone that officially doesn’t exist. Your phone will probably show the wrong time. Welcome to Australia.
The International Date Line and how long a day lasts
The date line roughly follows the 180 degree meridian of longitude but zigzags wildly to avoid splitting countries. It’s not defined by any international treaty – it’s a convention, and individual nations can (and do) choose which side of it they sit on.
Here’s a thought experiment. The earliest time zone on Earth is UTC+14 (Kiribati’s Line Islands). The latest is UTC-12 (Baker Island, an uninhabited US territory in the Pacific). When it’s midnight starting Monday in Kiribati (UTC+14), that’s Sunday 10:00 UTC, which is Saturday 22:00 in Baker Island (UTC-12) – still Saturday evening. The gap between the earliest and latest time zones is 26 hours. That means the calendar date “Sunday” exists somewhere on Earth for a total of fifty hours – from the moment it first appears in Kiribati to the moment it finally vanishes in Baker Island.
In 2011, Samoa jumped from UTC-11 to UTC+13, skipping December 30 entirely. The reason was trade: Samoa’s biggest partners are Australia and New Zealand, and being nearly a full day behind them meant losing two working days each week to the date line. By jumping forward, Samoa aligned its business week with its neighbours. An entire day – a Friday – simply ceased to exist.
Time zones aren’t even fixed through time
Here’s the part that really breaks your brain: the offset for a given place isn’t constant. It changes over the years. Not just because of daylight saving – the standard offset itself shifts when governments decide it should.
Take Perth. Right now we’re on UTC+8, Australian Western Standard Time. Straightforward. But Perth hasn’t always been UTC+8. Before 1895, Western Australia used local mean time – roughly UTC+7:43, based on the longitude of the Perth Observatory. In 1895 the colony adopted a standard offset of UTC+8. Then during both World Wars and again from 2006 to 2009, Perth observed daylight saving time and temporarily became UTC+9. During those DST periods, a timestamp from Perth at 2:00 AM on a transition day is ambiguous – did it happen before the clocks went back, or after? The same wall-clock time occurred twice. And when clocks spring forward, an hour simply doesn’t exist – 2:00 AM to 2:59 AM never happened. Anyone born in that hour, any event scheduled in that hour, any log entry timestamped in that hour: none of it is real.
This isn’t unique to Perth. Virtually every inhabited place on Earth has changed its UTC offset at least once. Some have changed it dozens of times. Russia has reshuffled its eleven time zones repeatedly – most recently in 2014, when several regions shifted by an hour. Turkey moved from UTC+2 to UTC+3 permanently in 2016, abolishing DST by simply staying on summer time forever. North Korea created its own timezone, UTC+8:30, in 2015, then switched back to UTC+9 in 2018 as a diplomatic gesture toward South Korea. Morocco has adopted, abandoned, and re-adopted DST multiple times, and currently observes it year-round except during Ramadan, when they temporarily suspend it – meaning the offset changes on religious dates that shift by roughly eleven days each year against the Gregorian calendar.
The IANA timezone database – the file your phone and your servers use to figure out what time it is – tracks all of this. Every historical offset change, every DST transition, every political decision that moved a clock. The database is updated several times a year because governments keep changing the rules. The entry for Australia/Perth alone contains the 1895 adoption, the wartime DST periods, and the 2006-2009 trial. If you’re writing software that handles time, this database is your source of truth – and the fact that it needs regular updates tells you everything about how stable time zones actually are.
The consequence for software is brutal. You cannot store a local time and assume you know the UTC equivalent without also knowing which version of the timezone rules were in effect. A timestamp of “2:30 AM, 25 March 2007, Perth” is meaningless unless you know whether DST was active – and the answer depends on whether you’re using the pre-trial rules or the trial rules. The time itself depends on when you ask the question.
Daylight saving time
And then there’s daylight saving time, which deserves its own category of complaint.
The idea is attributed to George Vernon Hudson, a New Zealand entomologist who proposed it in 1895 because he wanted more daylight hours after work for collecting insects. (The things that change the world.) Germany was the first country to actually adopt it, in 1916, to save coal during World War I. Britain and the United States followed.
When it happens varies wildly. The EU switches on the last Sunday of March and October. The US switches on the second Sunday of March and the first Sunday of November – a change made by the Energy Policy Act of 2005, which took effect in 2007 and broke a surprising amount of software. Australia varies by state. And because Australia is in the southern hemisphere, the transitions go the opposite way – clocks spring forward in October and fall back in April, which confuses anyone used to the northern pattern.
Where it doesn’t happen is a longer and more entertaining list. Most of Africa. Most of Asia. Iceland. Hawaii. Most of the tropics. Queensland, Australia – though New South Wales, Victoria, and South Australia, which share the same longitude, do observe it, leading to the odd situation where crossing a state border changes your clock. Here in Western Australia, we’ve voted against DST in four separate referendums – most recently in 2009, after a three-year trial – and the answer is always no.
And then there’s Arizona. Arizona doesn’t observe DST. But the Navajo Nation, which sits inside Arizona, does. And the Hopi reservation, which sits inside the Navajo Nation, doesn’t. Drive across those borders and your clock changes, doesn’t change, changes, and doesn’t change again. It’s a time zone nesting doll. Meanwhile, Lord Howe Island, a small Australian territory in the Tasman Sea, shifts by only 30 minutes for DST – because, apparently, why not.
The costs are real. A 2008 study by Janszky and Ljung in the New England Journal of Medicine found that heart attacks increase by about 5% in the week after the spring-forward transition, likely due to sleep disruption. Car accidents increase. Productivity drops. Software bugs bloom. The EU Parliament voted in 2019 to abolish DST entirely, but member states couldn’t agree on whether to keep permanent summer time or permanent winter time, and the proposal stalled.
If you’re writing code that handles time zones, the IANA tz database – sometimes called the Olson database, after its creator Arthur David Olson – is your scripture. It’s updated several times a year because governments keep changing the rules. I’ve written about the kind of compound complexity this creates in The Value Is in Ideas, Not Code – your library of knowledge about edge cases like these is exactly the sort of thing that separates useful software from software that crashes on a Sunday in Samoa.
Even the source of truth gets it wrong
The tz database is the closest thing we have to a global authority on time. Every phone, every server, every programming language runtime uses it. And it has been wrong.
Governments don’t give notice. Egypt has announced DST changes with literally days of warning – not enough time for the database to ship an update, propagate through OS vendors, and reach the devices that need it. In 2014, Egypt cancelled DST with ten days’ notice, then reinstated it two years later, then cancelled it again. Each flip left a window where every computer in Egypt was showing the wrong time. Morocco’s Ramadan DST suspensions are worse – they shift against the Gregorian calendar by roughly eleven days each year, so the database has to predict Islamic calendar dates in advance. Sometimes the prediction is wrong and a correction has to be issued after the fact.
Turkey, 2016. The government announced permanent UTC+3 with almost no lead time. Software using cached or bundled tz data – which is most software – was simply wrong until updates shipped. Java, Python, every major OS: all had a window where timezone calculations for Istanbul were incorrect. If you’d scheduled a meeting in Turkey during that window, your calendar was lying to you.
The lawsuit that nearly killed it. In 2011, a company called Astrolabe Inc. sued Arthur David Olson for copyright infringement, claiming the database incorporated data from their copyrighted timezone atlas. The database was briefly taken offline – the world’s timezone source of truth, gone. ICANN stepped in, took over maintenance under the Internet Engineering Task Force, and the lawsuit was eventually dismissed. But for a period, the infrastructure that every computer on Earth depends on for knowing what time it is was legally threatened by a copyright claim.
Historical corrections happen constantly. The database relies on historical records – newspaper clippings, government gazettes, personal recollections – that are sometimes incomplete or contradictory. Corrections to decades-old entries appear in updates regularly. Someone finds a gazette proving that a city changed its clocks on a different date than the database recorded, and a patch goes out.
And then there’s Antarctica. The South Pole doesn’t have a natural timezone – every line of longitude converges there, so the concept is meaningless. The Amundsen-Scott South Pole Station uses New Zealand time (UTC+12/+13) because its supply flights come from Christchurch. But other Antarctic stations use the timezone of their home country, or the timezone of their supply base, or whatever the station commander decided that year. The Antarctica/Vostok entry in the tz database has been corrected multiple times – at one point it referenced the South Magnetic Pole, which had drifted hundreds of kilometres away from the station. Vostok officially uses UTC+5 (matching its Russian supply base in Novosibirsk), but in practice the station has used UTC+6 and UTC+7 at various points depending on who was running the base that year. When the tz database maintainers tried to nail down the correct offset, the answer was: it depends on who you ask and when you asked them.
The tz database is maintained by volunteers. It’s one of the most critical pieces of infrastructure on the internet – right up there with DNS root servers and the BGP routing tables – and it runs on the goodwill of people who care about getting the time right. Every time your phone silently adjusts for a timezone change you didn’t know about, that’s someone on the tz mailing list who noticed, researched it, wrote a patch, and got it merged. The system works. It just works by the thinnest of margins.
So what time is it?
That’s the human story of time – thousands of years of sticks in the ground, springs and pendulums, political compromises, and a volunteer-maintained database that quietly keeps the world’s clocks from lying to us.
But we’ve been talking about time as something we agree on. What about time as something we measure? What is a second, really? How do you count one precisely enough to land a spacecraft or synchronise a financial transaction across continents?
That’s what How Clocks Work is about – the physics of the second, from quartz crystals to caesium atoms to the extraordinary clocks that won’t lose a second in the lifetime of the universe.