Every four years, we get a February 29 on the calendar – a date that often raises questions. Why do we need this extra day? And for that matter, how exact are the hours, minutes, and seconds ticking by on our clocks? It turns out, keeping time “on time” is trickier than it looks. From the Earth’s not-quite-even orbit to its wobbly rotation, and from centuries-old calendar fixes to modern clock adjustments, we’ve had to invent clever hacks – leap years, leap seconds, time zones, daylight saving – to keep our calendars and clocks aligned with reality. In this article, we’ll explore leap years and the little imperfections in our time units in a conversational journey through both science and history.
We usually say a year is 365 days. But astronomically, a year is defined by Earth’s complete orbit around the Sun – and that doesn’t tidy up to an integer number of days. In fact, a tropical year (the cycle of seasons from one spring equinox to the next) is about 365.2422 days long (Leap Years). That’s roughly 365 days, 5 hours, 48 minutes, and 46 seconds. This little fraction of a day may not sound like much, but it adds up. If we ignored it and made every year 365 days, our calendar would drift about 0.24 days (nearly 6 hours) off each year. After a century, the seasons would shift by about 24 days (Leap Years) – imagine celebrating midsummer in what used to be spring!
To solve this, we insert an extra day occasionally – that’s the leap day, February 29, which occurs (almost) every four years. By adding one day roughly every four years, we account for that extra 0.2422 of a day per year. Leap years keep our calendar in sync with Earth’s orbit so that, for example, June stays a summer month (in the northern hemisphere) and doesn’t drift into winter over time (What Is a Leap Year? | NASA Space Place – NASA Science for Kids).
The concept of adding leap days dates back to ancient times. Julius Caesar’s astronomers realized the 365-day year was too short, so in 46 BCE they introduced the Julian calendar with a leap year every 4 years (making an average year 365.25 days long). This was a huge improvement, but still not perfect: 365.25 days is a bit longer than the true year. The error was about 11 minutes per year, which doesn’t sound bad, but over centuries it accumulates (Give Us Our Eleven Days | The English Calendar Riots of 1752) (Give Us Our Eleven Days | The English Calendar Riots of 1752). By the 1500s, the calendar had drifted around 10 days out of alignment with the seasons (Gregorian calendar - Wikipedia).
To fix this, Pope Gregory XIII introduced the Gregorian calendar in 1582. The Gregorian reform did two things:
Dropped 10 days from the calendar to realign dates with the seasons (in 1582, Thursday October 4 was followed by Friday October 15, instantly correcting the drift (Gregorian calendar - Wikipedia)).
Adjusted the leap year rule: It kept leap years every 4 years, but made exceptions at century years. Under Gregorian rules, a year that is divisible by 100 is not a leap year, unless it’s also divisible by 400 (Gregorian calendar - Wikipedia). In other words, 1700, 1800, 1900 were not leap years, but 2000 was (because 2000 is divisible by 400) (Leap Years) (Leap Years). This tweak means we skip three leap days every 400 years, giving an average year length of 365.2425 days – extremely close to the actual 365.2422-day year (Gregorian calendar - Wikipedia) (Leap Years).
The result? The Gregorian calendar is off by only about 26 seconds per year (half a minute) (Leap Years). That tiny error will take around 3,300 years to accumulate to a full day’s drift (Leap Years). Not perfect, but pretty amazing for a system devised in the 16th century! So far, we haven’t needed any further tweaks (though by the year 4900 or so, our descendants might have to delete a leap day to fine-tune the calendar again).
Fun fact: The calendar switch wasn’t instant worldwide. Catholic countries adopted Gregory’s calendar in 1582, but others took longer. Britain and its American colonies, for example, held out until 1752. By then they needed to drop 11 days to catch up. In 1752, people in England went to sleep on September 2 and woke up on September 14 – September 3–13 simply didn’t exist that year (Give Us Our Eleven Days | The English Calendar Riots of 1752)! Some folks reportedly panicked, thinking the government had “stolen” 11 days of their lives (February 30 Was a Real Date). (Despite tales of “calendar riots,” historical evidence for actual riots is thin, but the story reflects real public confusion and grumbling at the time.)
If a year isn’t a neat whole number of days, you might wonder: what about the day itself? We learn a day is 24 hours, each hour 60 minutes, each minute 60 seconds. Unlike the year, we don’t usually insert “leap hours” or “leap minutes” into our clocks. But that’s because we’ve defined hours and minutes in a way that sidesteps some imprecision – at least until we get down to the seconds (more on that soon).
Originally, an hour was defined by the Sun’s motion. Ancient Egyptians and others used sundials and divided daylight into 12 hours (and night into 12 hours), which led to our 24-hour day. But the length of solar hours varied with seasons until mechanical clocks standardized the hour to a fixed length. Today, an hour is just 1/24 of a mean solar day by definition, and a minute is 1/60 of an hour. These units are human conventions, rooted in ancient Babylonian base-60 math. They’re perfectly even by definition – there are always 60 minutes in an hour, 60 seconds in a minute – except when we deliberately fudge things for our convenience, like with time zones and daylight saving.
The 24-hour day works well if everyone sticks to their own local solar time. In the 1800s, each town kept time by the Sun (noon was when the Sun was highest). That was fine until rapid travel and communication shrank distances. By the 19th century, scheduling trains across different local times was a nightmare – the United States alone had over 300 local time zones in use by the 1880s! (Why Were Time Zones Created?) Every city’s clock differed, confusing travelers and causing train scheduling chaos.
The solution was standard time zones. In 1883, North American railroads adopted four standard time zones for the continental US (Why Were Time Zones Created?). And in 1884, an international conference established the system of global time zones starting from the Prime Meridian at Greenwich, England (Why Were Time Zones Created?) (Why Were Time Zones Created?). The world was divided roughly into 24 one-hour zones. This meant for example, all of New York would use “Eastern Time” instead of each town doing its own thing. Noon on the clock might not be exactly when the Sun is at its peak for every location in the zone, but it’s close enough and far more convenient.
However, even time zones aren’t perfectly uniform. Not every region sticks to the neat one-hour offsets from UTC (Coordinated Universal Time). Some zones offset by 30 or 45 minutes instead. For instance, India uses UTC+5:30 (five and a half hours ahead of UTC), and Nepal uses the quirky UTC+5:45 (Half Hour and 45-Minute Time Zones ) (Half Hour and 45-Minute Time Zones ). Newfoundland (in Canada) is UTC−3:30, and the Chatham Islands (New Zealand) are UTC+12:45. These half-hour and quarter-hour offsets are often historical compromises to better align with local solar time or political boundaries. And one of the largest countries, China, spans five geographical time zones but chooses to use one single time zone (Beijing Time) for the whole country – which means in far western China, “noon” by the clock can be late afternoon by the sun!
Time zones also introduce the concept of the International Date Line on the opposite side of the world from Greenwich. Crossing this invisible line can make you “time travel” by a day. Fly from Los Angeles to Tokyo, and you might skip a calendar day; fly back and you might repeat one. This has led to some interesting date-line adjustments. For example, the island nation of Samoa decided to switch which side of the date line it’s on in 2011 for economic reasons – they skipped December 30, 2011 entirely, jumping straight to December 31, to align their calendar with their Asian and Australian neighbors (Samoa skips a day and crosses international dateline). (Conversely, Kiribati in the Pacific moved the date line in 1995 so that the whole country would be on the same side, resulting in one part of Kiribati leaping ahead by a whole day (A History of the International Date Line - Kiribati/Samoa adjustments).) These changes are rare, but they illustrate how cultural and political choices can override the clock and calendar.
Another human twist to the clock is Daylight Saving Time (DST). This doesn’t come from astronomical imprecision but from our desire to better match our hours to daylight. Under DST, we set clocks forward by one hour in spring (effectively “losing” an hour of sleep that night, as 2:00 AM jumps to 3:00 AM) and set them back by an hour in autumn (creating a 25-hour day when 2:00 AM repeats). The idea is to shift human activity to capture more daylight in the evenings during summer.
DST was first implemented on a broad scale during World War I. Germany and Austria introduced the first nationwide DST in April 1916 to save energy (fewer hours of lighting needed in the evening) (Why Do We Have Daylight Saving Time?). Britain, France, and others soon followed (Why Do We Have Daylight Saving Time?). The practice was dropped after WWI, revived in WWII, and today many countries use DST, though many others do not (or have abolished it). Not everyone is a fan – changing the clocks twice a year can disrupt sleep and schedules. Studies have noted spikes in tiredness-related issues and even a slight uptick in heart attacks in the days after the spring forward switch, when people lose an hour of sleep. And of course, DST can cause funny scheduling quirks: for example, an appointment at 2:30 AM might never happen on the spring change day (since the clock skips from 2 to 3), or happen twice in the fall.
Contrary to popular belief, Benjamin Franklin didn’t invent DST (he merely jokingly suggested people wake up earlier to save candles) (Why Do We Have Daylight Saving Time?). The modern DST idea was proposed by George Vernon Hudson and William Willett in the late 19th/early 20th century, but it took the world wars to really push it into practice (Why Do We Have Daylight Saving Time?) (Why Do We Have Daylight Saving Time?). Today, DST remains controversial – some regions have opted out (for example, most of Arizona doesn’t observe it, nor do many equatorial countries), and there are periodic debates about abolishing it entirely. For now, though, many of us continue to “spring forward” and “fall back”, adding yet another wrinkle to humanity’s timekeeping.
We’ve handled the extra fraction of a day each year with leap years, and we’ve standardized hours and time zones for human convenience. But there’s another imperfection: the length of the day itself. We like to think a day is exactly 24 hours of 60 minutes each – 86,400 seconds. That’s how we define our civil time. But in reality, the Earth’s rotation is not perfectly constant. The length of a solar day can vary by a few milliseconds and, over long periods, is gradually getting longer.
Why? One big reason is tidal friction – the gravitational pull of the Moon (and to a lesser extent the Sun) creates tides and drags on Earth, which slows Earth’s rotation ever so slightly. Over millions of years, this effect has made days longer (in the age of dinosaurs, a day was only ~23 hours because Earth spun faster!). Today, measurements show that on average Earth’s rotation is slowing such that the day lengthens by about 1.4 to 1.7 milliseconds per century (Leap second - Wikipedia). That is tiny, but again it accumulates. In addition to this steady trend, there are irregular fluctuations: earthquakes can slightly tweak Earth’s rotation, and even shifts in air and ocean currents or the distribution of mass (like melting ice caps) can speed up or slow down the planet’s spin by fractions of a millisecond (Leap second - Wikipedia) (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News).
For a long time, these minuscule changes didn’t matter to anyone. But in the last few decades, we’ve entered the era of super-precise timekeeping with atomic clocks. Atomic clocks tick away based on the vibrations of atoms (cesium, in the standard case) and are incredibly consistent. In 1967, the world redefined the second in the International System of Units (SI) as a fixed number of atomic oscillations (9,192,631,770 oscillations of cesium-133, to be exact) to match the length of a second as it was around that time. The goal was a stable, universal time base. This atomic second is almost exactly the traditional 1/86,400 of a mean solar day – but not exactly, because Earth’s rotation is a moving target.
By the late 1960s, scientists noticed that if we let atomic clocks run without adjustment, they’d gradually diverge from the Earth’s day. To reconcile atomic time with the Sun-based civil day, we introduced the concept of the leap second in 1972. A leap second is to the second what the leap day is to the calendar: an occasional one-second adjustment to keep clocks in sync with Earth. Basically, if astronomical observations show that the Sun is drifting out of sync with our clocks (i.e. Earth’s rotation has slowed enough that the day is nearly a second longer than 86,400 atomic seconds), timekeepers pause the clock for one second. This happens typically on June 30 or December 31 of a year, and when it occurs, the clock will read 23:59:60 for that one extra second before ticking to midnight (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News) (Leap Second - What is it?). In other words, one minute will contain 61 seconds on a leap second day (Leap Second - What is it?).
Since 1972, 27 leap seconds have been added (as of the end of 2016) (EarthSky | No leap second for December 31, 2021). For example, a leap second was added on December 31, 2016 at 23:59 UTC, which meant 2016 officially had 366 days and 1 extra second (a memorable year for time geeks!). Most leap seconds were added in the 1970s–1990s when Earth’s rotation was slightly slower; lately, they have become less frequent (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News). The process is managed by the International Earth Rotation and Reference Systems Service (IERS), which announces leap seconds as needed. We never subtract a second… until maybe soon. Interestingly, in 2020 and 2021, Earth had some unusually short days (it actually sped up a bit) (Leap second - Wikipedia), leading scientists to discuss the possibility of a negative leap second (meaning we’d skip a second) if the trend continues (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News) (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News). This would make a minute with only 59 seconds, something that hasn’t happened so far.
Leap seconds ensure that “noon” as shown on an ultra-precise clock still more or less corresponds to the Sun crossing the sky. Without them, after many years our 6 AM could start looking like the middle of the night in terms of the Sun’s position. However, they come with headaches: because they’re irregular and announced only about six months in advance, they can mess with technology and software. There have been incidents where systems crashed or misbehaved at the leap second moment (for instance, some web platforms and Linux servers had troubles in the 2012 leap second) (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News). To avoid these issues, tech companies like Google have even developed techniques to “smear” time – spreading the extra second out over a day so it never hits all at once (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News).
Due to such complications, the world’s timekeeping authorities have debated phasing out leap seconds. In 2022, they agreed to eventually stop inserting leap seconds after 2035 (at least for about a century) (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News). If that plan holds, civil time (UTC) would gradually drift from solar time by a few seconds, and a future generation might do a larger adjustment (like a one-time leap minute) to catch up. It’s an ongoing compromise: nature’s time vs. our clocks.
All these adjustments – leap days, leap seconds, shifting clocks and calendars – have led to some interesting stories and oddities throughout history. Here are a few favorites:
February 30 really existed… at least twice. Normally, of course, February has only 28 days (29 in a leap year). But Sweden, in 1712, had a February 30! (February 30 Was a Real Date) (February 30 Was a Real Date) It happened because Sweden was transitioning from the Julian to Gregorian calendar in a very convoluted way. After some mistakes, they decided to revert to the old calendar temporarily, and to do so they added an extra leap day in 1712 – resulting in Feb 30, 1712 on the Swedish calendar. (They finally switched to Gregorian in 1753, when they dropped the usual 11 days.) The other instance was in the Soviet Union, which briefly experimented with a revolutionary calendar that had 5-day weeks and 30-day months; in 1930 and 1931, they printed calendars with February 30 as a working day (February 30 Was a Real Date) (February 30 Was a Real Date). However, the Soviet experiment didn’t last, and they never officially abandoned the Gregorian calendar, so February 30 disappeared again.
The “missing” 11 days of 1752 (or 1582): As mentioned earlier, when countries adopted the Gregorian calendar, they had to drop several days. People sometimes joke about those missing days. For Britain, it was 11 days in 1752 (Sep 3–13). For Catholic Europe in 1582, it was 10 days (Oct 5–14). There’s a legend that people rioted shouting “Give us back our eleven days!” – it’s uncertain if actual riots occurred, but it’s true the sudden date jump confused folks (February 30 Was a Real Date). Imagine your diary skipping from the 2nd to the 14th with nothing in between!
Century years can surprise you: Many people learned the “every 4 years” rule for leap years, but don’t realize the century exception. For example, 2100 is not that far off – and it will not be a leap year even though it’s divisible by 4 (since it’s divisible by 100 but not 400) (Leap Years) (Leap Years). So after 2096, the next leap day will be 2104. The last time this rule kicked in was the year 1900 (no leap day that year), and the next time will be 2100. If you’re planning to live a very, very long life, take note!
Birthdays on Leap Day: People born on Feb 29 are often called “leaplings” or “leap year babies.” They typically celebrate on Feb 28 or Mar 1 in common years. In some legal systems, a Feb 29 birthday might legally be considered Feb 28 in non-leap years. There are rare cases of families with generations of Feb 29 births, and statistically about 1 in 1,461 people are born on a leap day (since 1,461 days = 4 years including one leap day).
Longest day (or shortest): Normally, our longest day is 24 hours, but if you consider DST or leap seconds, some days are longer or shorter. When we “fall back” in autumn, that day effectively has 25 hours (say, from 1 AM to 1 AM again, with 1–2 AM repeated). And any day with a leap second has 24 hours and 1 second. So far, the record for longest day in recent times could be June 30, 2012 or December 31, 2016, which had 86,401 seconds instead of 86,400. Conversely, if a negative leap second ever occurs, we’ll have a day with 86,399 seconds (23 hours, 59 minutes, and 59 seconds on the clock). On the other hand, if you flew from, say, California to Japan on a certain date, you might experience a day only, say, 10 hours long before the date jumps – or if you fly the other way, a day that lasts 36+ hours. It’s all relative!
Precision vs. perception: The sidereal day – the time it takes Earth to rotate once relative to the stars – is about 23 hours 56 minutes, a bit shorter than the solar day (because Earth moves in its orbit while rotating). This doesn’t affect our daily life directly, but it’s a reminder that “day” can be defined differently. Also, only on a few days of the year is the solar day exactly 24 hours. Because of Earth’s elliptical orbit and tilt, some solar days are a bit longer or shorter (up to about 30 seconds difference) – but our clocks run on the average. If you’ve ever heard of the “equation of time,” that’s the difference between solar time and clock time, which is why the timing of sunrise/sunset shifts a bit over the course of the year even aside from seasonal day length changes (The 7 most bizarre facts about leap day - Big Think) (The 7 most bizarre facts about leap day - Big Think). For practical purposes, we average it out, and only astronomers and sundial enthusiasts usually worry about that.
In the end, what all these quirks show is that timekeeping is a blend of science and human agreement. We live on a planet that doesn’t orbit or rotate in neat round numbers, and so our calendars and clocks need occasional nudges to stay in sync with the Sun, moon, and stars. The leap year is a brilliant hack that’s been serving us for over two millennia, and the Gregorian calendar fix was a masterstroke that most of the world lives by today. Leap seconds are a more recent invention, a tiny tweak for the era of atomic precision. Time zones and daylight saving, meanwhile, show how we mold time to our social and practical needs.
Despite the imprecision in the natural cycles, our timekeeping systems allow us to plan our lives with confidence that noon will be roughly midday and June will be summertime (for one hemisphere at least). It’s a testament to human ingenuity that we’ve mostly conquered the messy motions of Earth – adding a day here, a second there, or shifting an hour – to make time as regular as we need it to be. So next time someone jokes about “finding time” or having an “extra hour” of sleep, you might tip your hat to the leap years and leap seconds that quietly keep our world on schedule. After all, time waits for no one, but with a little cleverness, we’ve learned how to catch up to it.
Earth’s orbital period and need for leap years (Leap Years) (Leap Years)
Gregorian calendar reform and leap year rules (Gregorian calendar - Wikipedia) (Leap Years)
Calendar drift and historical calendar changes (Gregorian calendar - Wikipedia) (Give Us Our Eleven Days | The English Calendar Riots of 1752)
February 30 and calendar anomalies (February 30 Was a Real Date) (February 30 Was a Real Date)
Time zones history and offsets (Why Were Time Zones Created?) (Half Hour and 45-Minute Time Zones )
Daylight Saving Time history (Why Do We Have Daylight Saving Time?) (Why Do We Have Daylight Saving Time?)
Earth’s rotation and leap seconds (Leap second - Wikipedia) (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News)
Leap seconds added and future of leap seconds (EarthSky | No leap second for December 31, 2021) (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News)
Effects of Earth’s rotation irregularities (A faster spinning Earth may cause timekeepers to subtract a second from world clocks | AP News)