Leap Day and Leap Years

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Podcast Transcript

Once every 1461 days, sometimes, we have a day on the calendar that we don’t normally have: Leap Day. 

This extra day is a necessity if our calendars are kept in sync with the orbit of the Earth around the Sun, but it also can cause problems with people and computers. 

Nonetheless, whatever problems it might cause are far less than some of the alternatives, which would require leap weeks and even leap months. 

Learn more about leap years and the significance of February 29 on this episode of Everything Everywhere Daily. 

If you’ve listened to this podcast long enough, you know that I have a thing for calendars. I’ve done episodes on everything from how we got the days of the week to the Gregorian calendar to the months. 

So, there is no way I was going to let February 29th go by without doing an episode on it. Especially considering that this is the first February 29th that I’ve experienced since starting this podcast. 

I’ve touched on why there is a February 29th in previous episodes, but let me briefly restate the reason for it here. 

The problem is that our basic units of time, the day and the year, are determined by astronomical events, which are not evenly divisible by each other. 

I’ve read many proposals for different calendar systems, and while some of them do have their merits, they can’t escape this fundamental problem. Days do not divide evenly into a solar year. 

Our current calendar, the Gregorian calendar, is based on a solar year. That means it tries to get the seasons and the solar events, such as the solstices and the equinoxes, at the time of each year. 

However, the Gregorian calendar was a modification of the Julian Calendar, and the Julian Calendar was designed to replace the Roman calendar, which was truly awful. 

I’ve done a lot of episodes on Roman history and how many things in our modern world can be traced back to ancient Rome. However, the original ancient Roman calendar was well and truly one of the dumbest things ever invented.

The original Roman calendar had ten months, the first of which was March and the last of which was December. The word December means the tenth month, which is very odd considering it is now the 12th month. 

But between December and March, there was….. nothing. There were 61 days total unaccounted for in the calendar. Again, I’m not sure how you develop a calendar and not account for all of the days in the year, but the very early Romans did exactly that. 

Eventually, those 61 days were taken up by two more months, January and February, but there were still massive problems with their calendar. The calendar didn’t correspond to the solar year, and after several centuries, they celebrated the harvest festival in the spring. 

This was solved by Julius Caesar, who instituted a new solar calendar that solved the problems of the old Roman Calendar. The calendar of Julius Caesar, known as the Julian Calendar, was a pretty good calendar considering what it replaced. 

The calendar was actually the idea of Sosigenes of Alexandria. It had been known for centuries that a solar year was approximately 365 and ¼ days long. That quarter day was resolved in the Julian calendar by adding an additional day every fourth year. 

There would be 365 days every year, and then every fourth year, there would be 366 days to take care of that ¼ day that made up a year. 

I don’t want to rip on the Julian Calendar too much because it was a truly revolutionary development. However, the Julian Calendar wasn’t perfect. 

If you listened closely to what I said before, a solar year is approximately 365 and ¼ days long. The “approximately” part is key.

A more exact value is that a solar year is 365.2422 days long. That might not seem like much of a difference, but it became an issue after many centuries. Slowly, the calendar year began to misalign with the solar year. 

This error was resolved with a calendar which was championed by Pope Pope Gregory XIII. He proposed a new calendar that was very close to the Julian calendar but made a few modifications. 

To compensate for the errors in the Julian calendar, every year divisible by 100 would not be a leap year.  That would fix most of the problems, but it still left an error, so they made an additional change that if the year was evenly divisible by 400, then it would be a leap year. 

So the year 1900 was not a leap year, but the year 2000 was. 

That got the calendar pretty close to the solar year, close enough that it would correspond to the solar year for thousands of years. 

Just as an aside, it will probably be necessary to make a change to the calendar before the year 4000, such that any year evenly divisible by 4000 will not be a leap year. If you really want to get pedantic, they will probably also have to make another change such that if the year is evenly divisible by 20,000, then it is a leap year. 

The point of all of this is that the added day once every four years is sort of just a shim used on the calendar to make it work. It is technically known as an intercalary day. 

There have been many proposed calendars, but no matter the system that has been proposed, there has to be some sort of intercalary period somewhere to make the days fit a year if you want to make the calendar approximate a solar year. 

For example, a strict lunar calendar like the Islamic calendar has no intercalated period at all. That is why a holiday like Ramadan can fall in any month over a period of years. 

The Jewish Calendar is a lunisolar calendar. It follows the phases of the moon but also tries to keep the months at roughly the same time of year. They are able to do this because they have entire leap months to even things out. 

There are proposed calendars such as the Hanke–Henry Permanent Calendar. This calendar would put the number of days at 364, a number which is nicely divisible by many numbers. Under this calendar, every month would have 30 days, save for the last month in every quarter, which would have 31.

It would also then allow for every day on the calendar to correspond to the same day of the week every year. So, in the Hanke–Henry Permanent Calendar, January 1 would always be on a Monday every year. 

However, this calendar would be off by approximately one ¼ days every year. 

To get around this problem, every five or six years, there would be an entire leap week. This leap week wouldn’t belong to any month; it would just be its own thing.

So, we got an extra day every four years, and it is something that we are going to have to deal with.

For the most part, adding a day every four years is not a problem for most computers. In fact, adding one day to an already existing month is computationally much easier than a leap week or a leap month that isn’t associated with any current month.

That being said, every four years, there are computer programs that fail when February 29th arrives. Most programming languages have libraries you can reference that can handle leap year just fine, but sometimes they are used incorrectly, or the programmer just forgets about leap year. 

In 2016 alone, a host of bugs were found on many devices. 

Cars made by the Jeep corporation had their clocks reset automatically on February 29 because they couldn’t handle the date. 

The Düsseldorf International Airport had a problem with its bag handling system, which resulted in 1200 pieces of luggage not getting on its planes.

The United States Postal Service had issues where their website couldn’t handle February 29. 

Apple’s iOS and WatchOS had an issue where their calendar preview feature didn’t work on leap day. 

These are all pretty minor issues, but it’s one of those things that constantly creeps up every year. 

One of the longest-running leap-year bugs in computing has to do with Microsoft Excel. Ever since Excel was released for the Macintosh in 1985, it had a bug where it thought the year 1900 was a leap year. 

The reason for the bug is that Excel was designed to be backward compatible with Lotus 1-2-3, which was the dominant spreadsheet program at the time, and Lotus 1-2-3 had that bug.

In almost 40 years since Excel was on the market, the bug has never been fixed. Not only has the bug never been fixed, but the bug has been codified in the Ecma Office Open XML specification.

If you create a spreadsheet that uses dates from the year 1900, it will create a date, February 29, 1900, that never existed.

The other big issue with leap years is the reoccurring events that can happen during them: birthdays. 

I had an uncle who was born on February 29, and I have another friend who was a leap-year baby as well. The odds of being born on February 29 is one in 1,461.

Most people with birthdays on February 29th will usually celebrate it on February 28th or March 1st because that would be approximately when one year has passed. However, they would only have a birthday anniversary every four years. 

The issue of leap year birthdays was a significant plot point in the Gilbert and Sullivan musical The Pirates of Penzance. In it, Frederic, the pirate apprentice, discovers that he is bound to serve the pirates until his 21st birthday. In the play, that is interpreted by the pirates to mean he is bound to serve until he turns 88 years old since 1900 was not a leap year, rather than until his 21st year.

Each legal jurisdiction has different rules regarding how they handle February 29th. 

Generally speaking, in the United States, when things are defined in terms of days, February 29th is just treated as a regular day. For example, if you had a non-compete clause that lasted one year and began on March 1st, and the next year was a leap year, it would last for 366 days, not 365.

However, if things are defined in terms of years, then leap years are usually ignored because they aren’t relevant.

There are a host of cultural traditions associated with February 29th. 

According to tradition, in 5th-century Ireland, St. Bridget lamented to St. Patrick that women were not allowed to propose marriage to men. So St. Patrick allowed February 29th to be the one day when women could propose marriage or ask a man to dance. In some places, February 29th became known as Bachelors Day for this reason. 

This tradition was mostly held in Ireland and Scotland, but it also was celebrated in the United States in the 19th century.

February 29th and Leap Year are necessary to make our calendars function, but this necessity always ends up with computer bugs, misunderstandings, and unique circumstances every four years. 

The Executive Producer of Everything Everywhere Daily is Charles Daniel. 

The associate producers are Peter Bennett and Cameron Kieffer. 

Today’s review comes from listener D with a tilde, over on Apple Podcasts in the United States. They write:


I finally did it! I listened to hundreds of episodes on double-speed, and I met my goal of joining the Completionist Club before the start of the Super Bowl. I love the show. This podcast satisfies my desire to learn something new every day while also not taking too long, as I listen to lots of other podcasts as well. It’s one of a few podcasts that I insist on listening to on the day of release. Looking forward to hundreds more episodes. ¡Salud!

Thanks, D with a Tilde! I’d like to formally welcome you to the completionist club. As always, please make sure you show your card at the door and make sure to tip the concierge. 

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