The Julian and Gregorian Calendars

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What is today’s date? 

If you answer that question, most likely you are giving an answer based on a calendar that goes all the way back to one put in place by Julius Caesar. 

Caesar’s calendar, aka the Julian Calendar, was pretty good, but it developed problems over time, so it was modified in 1582 by Pope Gregory XIII. 

This calendar, the Gregorian Calendar, what we’ve been using for the last several hundred years, and it works pretty well.

Find out how the calendar of a Roman ruler and a Renaissance Pope came to be on this episode of Everything Everywhere Daily. 


To understand how our current calendar came to be, we have to go back to Ancient Rome. Not just the Roman Empire or the Roman Republic, but way back to the Kingdom of Rome.

The calendar used by the Romans at that time bears a resemblance to our calendar, but it was radically different. Legend has it that the first calendar was created by the first king of Rome and its founder, Romulus in 753 BCE.

This first Roman calendar had 10 months, each consisting of 30 or 31 days.

The months were:

  • Martius 
  • Aprilis 
  • Maius 
  • Junius 
  • Quintilis 
  • Sextilis 
  • September 
  • October 
  • November 
  • December 

You will recognize most of the names of the months because they are pretty much what we use today. The first four months were named after Roman gods, and the last six were just the number of the month. Also, their first month was March.

The problem was this calendar wasn’t even close to being 365 days. It only had 304 days, and the remaining 61 days weren’t really even assigned to any month, which sort of defeats the entire purpose of having a calendar. 

The second King of Rome Numa Pompilius, made some major changes to the calendar to solve some of the obvious problems.

First, he added two months after December to take care of the unassigned dates. The new months were Januarius and Februarius. He also shifted around the number of days in each month, such that the year had 354 days.

This doesn’t quite match up to the 365 days in a year, so they added a full leap month every few years called Intercalarius that had 27 days. In the years with Intercalaris, the year would have 377 days. 

This was better than having a bunch of mystery days, but there was no set rule to when they would have a leap year, and they would have to have it about every two or three years.

For several centuries this calendar is what was used in Rome, with the only real change being the change from March to January as the first month of the year. Oddly enough, even though they changed the numbering of the months, months such as September, October, November, and December, which mean 7th, 8th, 9th, and 10th, didn’t change their names. 

Over time, the years got really out of whack, and the Romans were celebrating their harvest festival before they started planting. 

In 45 BCE Julius Caesar decided to put an end to this nonsense and eliminated the leap month of Intercalaris and narrowed it down to a single extra day once every three years. 

This calendar looks very similar to ours today, save for the number of days in each month. February, for example, had 29 days, with 30 in a leap year. Months alternated between 31 and 30 days. 

The final fixes to the calendar which became known as the Julian Calendar were put in place under Emperor Augustus. He renamed the month of Quintilis to July, in honor of Julius Caesar, and renamed Sextilis to August, after himself. 

He also established the number of days in each month to what we have today and also set leap to occur once every four years instead of every three. 

All things being equal, the Julian Calendar did a pretty good job. It was the best solar calendar in the world, and it was accurate to within 11 minutes per year. 

Here I should explain the problem in all calendar systems. The number of days in a year isn’t a round number. The Julian Calendar works on the assumption that there are 365.25 days in a year. This is almost right, but it isn’t quite right. There are actually 365.2422 days in a year. 

That small difference didn’t really matter to the Romans or to anyone who came immediately after them, but it did start to make a difference over the centuries. That difference is about 3 days every four hundred years. 

By the time the 16th century came around, the calendar was off by 10 days from where it should have been. This was causing problems with where Easter landed. 

This divergence with the calendar date and the spring equinox had been noticed throughout history. The Venerable Bede noticed in the 8th century that the date was off by more than 3 days. Roger Bacon noted at the beginning of the 13th century that the difference was 7 or 8 days. Starting in the 14th century there was a visible need for reforming the calendar to get things back on track with the seasons. 

The plan for fixing the calendar was developed by an astronomer by the name of Aloysius Lilius. He realized that if the calendar was off by 3 days every four hundred years, the simplest thing to do would be the only have 97 leap years, every four hundred years. 

Thus, he created the simple rule that changed the Julian Calendar into the Gregorian Calendar. 

A leap year shall be every four years unless the year is divisible by 100, but those years that are divisible by 100 are leap years if it is divisible by 400. 

By adopting this system of leap years, you reduce the error in the length of a year down from 11 minutes per year to just 26 seconds per year. 

Changing the system of leap years every century wasn’t really a big deal. Getting the calendar back to where it was supposed to be, was a very big deal. It would require a one time jump ahead in the calendar, to compensate for the drift in the Julian Calendar over the centuries.

In 1582, Pope Gregory XIII bit the bullet and set in place the new leap year change, but also said that they were going to jump ahead 10 days to correct the Julian Calendar. On October 4th, 1582, people went to bed and they woke up on October 15th.  October 15th, 1582 can be considered the first day the Gregorian calendar was in effect. 

However, it didn’t take effect everywhere. Technically, the Pope only had the power to change the calendar in the Papal States, where he ruled. In Catholic regions such as the rest of the Italian peninsula, France, Poland, Spain, and Portugal adopted it right away. 

Other countries in Europe resisted. Orthodox Countries like Greece and Russia didn’t want to change because the reform came from the Catholic Church. Protestant countries like England and Sweden didn’t want to change because the reform came from the Catholic Church.

The adoption of the new calendar took several centuries. The longer some countries waited to change, the more days they had to adjust for. The initial change in 1582 was 10 days, but it was 11 days after 1700, 12 after 1800, and 13 after 1900, which is the difference today. 

Most of Germany, Switzerland, Denmark, and Norway came on board in 1700. 

England, and by extension the United States, adopted the Gregorian Calendar in 1752. 

Russia, Romania, Bulgaria, and Greece didn’t adopt it until the early 20th century. 

The most recent country to adopt it was Saudi Arabia in 2016. They were on the Islamic lunar calendar but moved to the Gregorian calendar as a cost-saving measure. Because there are 11 more days in the Gregorian calendar than the Islamic calendar, that meant they could pay civil servants the same annual salary, but get 11 more days of work out of them. It was basically a pay cut in the form of calendar reform. 

One odd case of transitioning was Sweden. Sweden was originally going to gradually change over the course of 40 years by just not having 11 leap years from 1700 to 1740. It would have left them in a state where their calendar wouldn’t have been in sync with anyone else for 4 decades. 

Their transition wasn’t managed very well, so after a few years, they decided to go back to the Julian calendar. To do this, they had to add 2 days, which they did at the end of February 1712. They had two leap days, including the only February 30th in history. 

They eventually bit the bullet and did the change all at once in 1753. 

One problem that the calendar change brought about was how to date things that happened before the change was implemented. The classic example in the United States was George Washington’s birthday. 

George Washington was born on February 11, 1731, in the English Calendar which started the year on March 25 (yes, their year started in the middle of the month, which was also corrected when they changed calendars). Washington’s birthday is now recognized as February 22, 1732. This can lead to problems for historians trying to date events using original documents, especially considering that different countries adopted the calendar at different times. 

I noted before that the Gregorian Calendar changed the error in the length of a year from 11 minutes per year to only 26 seconds. You might be wondering, 26 seconds isn’t zero, so might we still have a problem at some point in the future?

The answer is yes, but thankfully the solution is pretty easy. The current error leads to a drift of about 1 day every 4,000 years. The astronomer John Herschel, son of William Herschel the discoverer of Uranus, in the 19th century proposed a simple addition to the Gregorian leap year rule that would solve most of the problem. 

If the year is divisible by 4,000, then it isn’t a leap year. 

That means that the calendar would remain exactly the same, except that the year 4,000 wouldn’t be a leap year. 

Adopting this rule would bring the error down to one day per 20,000 years.

This reform has been proposed, but there has been no action taken on it, probably because we have 2,000 years to worry about it, and we have bigger issues to deal with.