What Time Is It? dealt with the hour – a fragile compromise between the sun and politics. The date next to it is fragile too, built from a different cast of characters: monks miscalculating epochs, popes deleting weeks, traders trying to align their working days with their trading partners, and software developers stuck with whichever epoch their operating system chose decades ago.
The fundamental problem
Calendars are hard for the same reason time zones are hard: the universe didn’t supply round numbers.
The Earth’s orbital period is roughly 365.2422 days. The lunar month is roughly 29.5306 days. Neither divides neatly into the other, and neither divides neatly into a day. Every calendar humanity has ever built is a compromise – some prioritise the sun, some the moon, some try to honour both, and a few just give up and count days from a fixed point.
There is no clean answer. There is only a choice about what to round, what to ignore, and what to patch with the occasional extra day or month bolted on.
The Gregorian calendar and the eleven missing days
The Gregorian calendar – the one most of the world uses for civil purposes – counts years from an epoch chosen in the 6th century by a monk named Dionysius Exiguus, who was trying to calculate the birth year of Jesus and got it wrong by several years. Modern scholarship places the actual birth somewhere between 6 and 4 BCE, which means the year on your phone is off by half a decade from the event it’s nominally counting from. We are stuck with Dionysius’s guess, because nobody is going to renumber every document, gravestone, and database in the world to fix it.
The Gregorian calendar uses a solar year: 365 days, with a leap day every four years, except every hundred years, except every four hundred years. So 1900 was not a leap year, but 2000 was. The rule keeps the calendar aligned with the seasons to within roughly one day every 3,236 years – close enough that nobody currently alive needs to worry about the next correction.
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 fix required a one-time correction: ten days were simply deleted. In the countries that adopted the new calendar in 1582, October 4 was followed by October 15. Nine days that never happened.
The switchover was not smooth. Catholic countries adopted it immediately. Protestant and Orthodox countries dragged their feet for centuries. Britain didn’t switch until 1752, by which point the discrepancy had grown to eleven days. September 2, 1752 was followed by September 14. Rents, wages, and birthdays had to be renegotiated. There’s a persistent legend that mobs rioted in the streets shouting “give us back our eleven days!” – the truth is more prosaic; the political turmoil was real but largely quiet.
Russia held out until 1918. Greece until 1923. For centuries, different countries were on different dates at the same time. The October Revolution? It happened on 25 October by Russia’s Julian calendar – 7 November by the Gregorian calendar everyone else was using. An October revolution that happened in November.
Lunar, solar, lunisolar
Beyond the Gregorian, the variety is dizzying.
The Islamic (Hijri) calendar is purely lunar – 354 or 355 days per year, so its months rotate through the Gregorian seasons over a 33-year cycle. Ramadan slowly walks through the year, falling in summer for a while, then spring, then winter. Months traditionally begin when a crescent moon is physically sighted by human observers. Not calculated – observed. Saudi Arabia and Morocco sometimes start Ramadan on different days because one country’s observers spotted the crescent and the other’s didn’t. The date of the most important month in the Islamic calendar is, in the strictest traditional sense, unknowable in advance. Software that has to schedule Islamic holidays falls back on calculated approximations and accepts that it will sometimes be a day off.
The Hebrew and Chinese calendars are lunisolar: lunar months adjusted with the occasional leap month to stay aligned with the solar year. The Hebrew calendar uses a 19-year cycle in which seven of the years contain an extra month. The Chinese calendar uses astronomical calculation to decide when to insert a leap month based on solar terms. The result is months that follow the moon and years that follow the sun, glued together by a rule that sounds simple and is anything but.
The Hindu calendars – there are several regional variants – are also lunisolar but use different epoch dates and different rules. The Bengali calendar starts its year in mid-April. The Tamil calendar uses a 60-year cycle of named years. None of them agree with each other, and most have to be reconciled with the Gregorian calendar for civil purposes.
The year itself is negotiable
The number on your screen depends on which civilisation you ask.
The Ethiopian calendar runs seven to eight years behind the Gregorian – a result of using a different calculation for the Annunciation. Ethiopia entered the third millennium in 2007 by Western reckoning. The country celebrated. Most of the world didn’t notice.
The Thai Buddhist calendar counts from the death of the Buddha in 543 BCE, which is why Thai expiry dates look like they’re from the future. A bottle of water bought in Bangkok in 2026 might be stamped with an expiry of 2570. The product hasn’t time-travelled. The calendar just starts somewhere else.
The Juche calendar in North Korea counts from Kim Il-sung’s birth year (1912) – introduced in 1997, three years after his death, retroactively renumbering the entire country’s history. The calendar exists alongside the Gregorian in official documents, with the Juche year cited first.
The Hebrew calendar counts from a calculated date for the creation of the world. We’re currently in the year 5786 by that reckoning. The Islamic calendar counts from the Hijra – Muhammad’s migration from Mecca to Medina in 622 CE – placing us in the 1440s. The Republic of China calendar still in official use in Taiwan counts from the founding of the republic in 1912, making 2026 the year 115. None of these civilisations is wrong. They are answering a slightly different question.
Calendars without numbers
Not all calendars count days at all in the way Western calendars do.
Here in Western Australia, the Nyoongar people – the traditional custodians of the south-west – use six seasons based on ecological indicators rather than calendar dates. Djilba (first rains) starts when the first rains come, which might be August or September depending on the year. Bunuru (the hot, dry time) starts when the weather turns, not when February begins. You know what season you’re in by looking at the land, not at a calendar. It’s a fundamentally different relationship with time: not “what date is it?” but “what is country doing right now?” (“Country” in Aboriginal English means the land itself – the living landscape, not a nation state.) It’s a calendar that’s always in sync with the actual ecology, at the cost of being impossible to print on a wall planner.
Other Indigenous calendars across Australia work similarly. The Yolngu people of Arnhem Land recognise six seasons based on wind direction, plant flowering, and animal behaviour. The D’harawal of the Sydney basin recognise six. None of them line up with the Gregorian quarters because the Gregorian quarters describe northern-hemisphere agriculture, not the actual rhythms of the southern continent.
Roman counting and revolutionary weeks
Not all calendars even count the same direction.
Roman calendars counted backwards from fixed points in each month – the Kalends (the first), the Nones (around the fifth or seventh), and the Ides (around the thirteenth or fifteenth). Caesar was assassinated on the Ides of March – March 15 – but a Roman would have referred to the day before that as “the day before the Ides” rather than “the fourteenth.” Days were named relative to the next landmark, not numbered absolutely.
The Maya Long Count tracked elapsed days from a mythological creation date in 3114 BCE, using a base-20 system with one quirky base-18 layer. It generated the apocalypse hysteria around December 21, 2012, when the count rolled over from one b’ak’tun to the next. The Maya themselves did not predict the world would end. They predicted the counter would tick. Western tabloids did the rest.
The French Republican Calendar (1793-1805) was a deliberate attempt to scrub Christianity and royalty out of the year. It introduced a ten-day week, three weeks per month, twelve months of thirty days, plus five or six “complementary days” tacked on at the end of the year. Months got new poetic names – Brumaire (mist), Thermidor (heat), Floreal (flowers). It was abolished after twelve years partly because workers only got one day off in ten, partly because nobody outside France used it, and partly because the calendar’s astronomical rules required astronomical observations from the Paris Observatory – which made it deeply impractical for shipping and trade.
The Soviet Union tried something similar in 1929 with a five-day week, with workers divided into five colour-coded groups so that production could continue uninterrupted – one fifth of the workforce was always on rest. Family members in different colour groups never had a day off together. The experiment was abandoned within a few years.
The International Date Line and the disappearing day
Once the world agreed on Greenwich as the prime meridian, an awkward consequence followed: somewhere on the opposite side of the world, the calendar date had to change. That somewhere is the International Date Line, which roughly follows the 180-degree meridian but zigzags wildly to avoid splitting countries.
It’s not defined by any treaty. It’s a convention, and nations choose which side they sit on.
The earliest time zone on Earth is UTC+14 (Kiribati’s Line Islands). The latest is UTC-12 (Baker Island and Howland Island, both uninhabited). The gap is 26 hours, which means any given calendar date exists somewhere on Earth for a total of fifty hours. New Year’s Eve starts in Kiribati and finishes more than two days later, by clock time, somewhere in the Pacific.
Kiribati earned its UTC+14 the hard way. Until 1995, the country straddled the date line: the western Gilbert Islands were on Tuesday while the eastern Line Islands were still on Monday. A government on the wrong side of its own date line struggles to function. Civil servants in the capital couldn’t telephone the eastern islands during normal business hours because the eastern islands were closed for the day before, or open for the day after. In 1995 the country redrew its time zone so the whole nation sat on the same side of the line, which meant the Line Islands skipped a day – December 30, 1994 simply did not happen there.
Samoa did the reverse in 2011. For more than a century Samoa had been on the American side of the date line (UTC-11) because most of its 19th-century trade was with California. By 2011, most of its trade was with Australia and New Zealand, which were a full day ahead. The mismatch meant Samoan businesses had only four overlapping working days per week with their main partners. The government decided to jump across the line. December 29, 2011 was followed directly by December 31. Friday December 30, 2011 simply ceased to exist in Samoa. People born on December 30 in earlier years had no birthday that year. The country switched from UTC-11 to UTC+13.
The neighbouring American Samoa, on the other hand, stayed where it was. The two Samoas are 100 kilometres apart and now live a full day apart by clock.
When computers count days
Every operating system, every database, every programming language has had to make peace with all of this. The compromise is usually a fixed epoch – a reference moment from which time is counted as a single number (typically seconds or milliseconds) – and a separate library that knows how to translate that number back into a human-readable date in a human-chosen calendar.
The choices are deeply arbitrary.
- Unix counts seconds from 1 January 1970 UTC. This is the most widely deployed epoch on Earth – inside almost every server, every smartphone, every embedded device. It was chosen because it was a recent round date when Unix was being designed, and nobody expected the choice to matter for very long. It will overflow a signed 32-bit integer in 2038 – the Year 2038 Problem, also known as the Unix Millennium Bug – which is currently a slow-burning crisis for any 32-bit system that hasn’t been updated.
- Windows uses 1 January 1601 (the start of the previous Gregorian 400-year cycle, chosen so that calendar arithmetic was simpler).
- macOS Cocoa uses 1 January 2001.
- GPS counts weeks from 6 January 1980. The week counter was originally 10 bits, which rolled over for the first time in 1999 and again in 2019. Receivers that didn’t handle the rollover started reporting times decades in the past.
- NTP uses 1 January 1900. Its 32-bit second counter will roll over in 2036, two years before the Unix problem.
- Excel uses 1 January 1900 as day 1, and famously believes 1900 was a leap year. It wasn’t – 1900 was divisible by 100 but not 400, so the Gregorian rule says no leap day. But Lotus 1-2-3 had the same bug, and Excel chose compatibility over correctness when Microsoft was trying to win the spreadsheet wars in the 1980s. That bug ships in every copy of Excel to this day. Any date arithmetic in Excel that crosses the (nonexistent) February 29, 1900 is silently wrong.
The pattern repeats. Every computer system makes a choice about its epoch, and every choice ages badly. If you store a date as “days since 1 January 1900” in 16 bits, you get to 2079 before you run out. If you store it as a Gregorian “year, month, day” triple, you’re fine for billions of years but you have to do calendar arithmetic every time you want to compute a duration. The choice between those two – a number, or a structured representation – is one of the oldest debates in software, and there is still no clean answer.
So what day is it?
The hour on your phone is a fragile compromise between the sun and politics. The date next to it is a fragile compromise between the moon, the sun, and several thousand years of calendar reformers, deleted weeks, regional epochs, and arbitrary numbering systems chosen by people who are now dead.
Today’s date depends on:
- Which calendar. Gregorian for civil purposes in most of the world, but Hebrew, Islamic, Chinese, Ethiopian, Thai, Juche, and dozens of others operate alongside it for religious or national use.
- Which side of the date line. And whether the date line in your part of the world has moved recently.
- Which epoch your computer was built on. And whether that epoch is about to overflow.
- Whether your timezone has changed recently. Russia has reshuffled its time zones repeatedly. Samoa moved itself across the date line. Kiribati skipped a day. Each of those events makes “what day was it on the 30th of December 1994 in the eastern Line Islands?” a question with an unsatisfying answer.
We’ve now covered the human story of the hour and the day. But all of this has been about how we agree on time. The next post asks what we’re actually measuring when we count seconds at all. Ticks or Tocks? is about the physics of the second – from quartz crystals to caesium atoms to optical lattice clocks that won’t lose a tick in the lifetime of the universe.