We all know that a leap year occurs every four years. But not always. It’s complicated. 

Photo by Andrea Piacquadio from Pexels

Humans have been devising calendars and methods of timekeeping for thousands of years. It’s our attempt to bring order, structure, and consistency to our lives. As civilizations grow more complex, calendars and timekeeping must be more precise. Our history of timekeeping gives us a fascinating glimpse of this evolution.


Some of the oldest known calendars come from the ancient Sumerians. The Umma calendar of Shulgi, a Sumerian king who reigned from about 2029-1982 BCE, is one of the earliest ever found. 

The Sumerians divided the year into twelve lunar months, giving them about 360 days. Aware that this caused a shortfall of days compared to the solar year, they usually added around 5 days of festivals and feasting at the end of the year.

The Babylonians took this method of timekeeper for themselves, also observing a 12 month lunar calendar. When the calendar no longer coincided with the proper seasons, an extra (or intercalary) month was added to bring it back into alignment. 

The Egyptian civil calendar dates back to about 2510 BCE and consisted of twelve months of 30 days each, totaling 360 days. Five intercalary days were added each year that were celebrated as the birthdays of the gods Osiris, Horus, Set, Isis and Nephthys. The regular months were grouped into three seasons according to the seasons of the Nile River.

It is probable that this civil calendar was based on the astronomical observations of the reappearance of Sirius in the morning sky at the same time the Nile reached its flooding stage in the 5th and 4th millennia. 

In 238 BCE, Ptolemy added a 6th intercalary day every four years to keep the civil calendar in alignment with the seasons.


Many ancient cultures used a purely lunar calendar. The moon’s orbit is about 29.5 days, so months usually alternated between 29 and 30 days each. Twelve lunar months total only 354 days, falling eleven days short of a solar year. An intercalary month would be added as needed to bring the calendar back into alignment with the seasons, but these additional months were usually inserted at random.

The oldest lunar calendar found was from Warren Field in Scotland. It dates back to 8000 BCE. 

As various cultures moved to a lunisolar calendar with intercalary days or months for their civil calendar, most still maintained a purely lunar calendar to determine religious and national holidays.


When Julius Caesar ruled the Roman Empire, the Romans observed a 355-day calendar and it was lagging far behind. The pontifices before home should have inserted eight intercalary months over the previous two decades, but due to political motivations, only five had been added.

Julian decided to fix the calendar so it could flow year after year without human intervention. With the help of Greek mathematicians and astronomers who borrowed ideas from the Egyptian calendar, Julian proposed his calendar reform. 

In order to line up the calendar with the solstices and equinoxes again, the year 46 BCE was 445 days long. It became known as the “Last Year of Confusion”. The reformed calendar began on January 1, 45 BCE.

The new calendar was 365 days long with a leap day added every four years. Intercalary months were abolished and the leap day was inserted in February, the shortest month of the year.

Yet even this calendar was flawed. By 1582 CE, nearly 1,600 years later, the calendar had set us ten days ahead and out of sync with the solstices and equinoxes again. That’s because the solar year is a few minutes less than 365.25 days. 


Photo from britannica.com

Pope Gregory corrected this with another calendar reform. Because the date of Easter is anchored to the date of the Spring Equinox, the Catholic Church wanted to ensure the calendar would no longer drift over time. 

The new calendar established rules for the leap year:

“A leap year is every 4 years unless the year ends in 00, but if the century is divisible by 400, it is still a leap year.”

So while the years 1600 and 2000 were leap years, the years 1700, 1800, and 1900 were not.

To correct the 10-day drift, Romans ended the day on October 4 and started the next day as October 15 with no changes to the day of the week. The New Year was reestablished as January 1. Although the Catholic countries immediately adopted this New Year, Protestant and Orthodox countries resisted. England and its colonies,including the United States didn’t conform until 1752, and Greece lagged behind until 1923! 

Read more about how March 25 used to be America’s New Year.

Today, nearly every country in the world uses the Gregorian calendar as their civil calendar.


As close as he was to getting it right, the Gregorian calendar is still 27 seconds faster than the actual solar year. That means that there will be a day of drift every 3,226 years. 

The Revised Julian Calendar makes up for this discrepancy and is the most accurate solar calendar in the world. Its rules are:

“Years divisible by 4 are leap years. However, years also evenly divisible by 100 and the remainder is not 200 or 600 when divided by 900 are NOT leap years.”

We don’t use this calendar because it wouldn’t make much of a difference. The Gregorian and Revised Julian calendars will remain in sync until 2800 CE. That will be a leap year on the Gregorian calendar, but not on the Revised Julian calendar.


The earth’s rotation on its axis has no relation to the time it takes to make a single trip around the sun. If timekeeping has shown us anything, it’s that humans are the inventors of time and we’ve been at it for thousands and thousands of years. Although we have invented time, we are all quite aware that we have a limited amount of it to enjoy each day on this beautiful Pale Blue Dot.