Leap Day 2020: A Brief History of Leap Day

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Today is February 29—Leap Day 2020! This date occurs once every four years. Why do we have this quadrennial practice? Most people grow up learning that the year is 365.25 days long, necessitating that most years are 365 days long, with every fourth year being 366 days long. However, the story is a bit more complicated than that.

Times Are A’changin’

Like most ancient calendars, the original Roman calendar was a lunar one, with months defined by the moon’s phases.

Our calendar dates to the Roman Empire. Like most ancient calendars, the original Roman calendar was a lunar one, with months defined by the moon’s phases. Lunar phases repeat with the synodic month. The synodic month varies between 29.18 days and 29.93 days, with an average just a little more than 29.5 days. Consequently, the Roman calendar had months that alternated between 29 and 30 days, though occasionally there were two 30-day months in a row. But this resulted in a year that was only 354–355 days long, about 10 days short of the year’s true length. To keep the calendar in sync with the true length of the year, an intercalary month was inserted approximately every third year. Since the custom was to begin the year with the vernal equinox, which is in March, the intercalary month usually was inserted before March, after February, normally the twelfth month on the Roman calendar. In the second century BC, the Romans moved the first of the year to January 1, though they continued the practice of inserting the intercalary month, when required, after February.

Who was to decide when it was necessary to insert an intercalary month? As with most ancient cultures, that responsibility fell to the priests. However, some Roman priests were open to corruption, and they often adjusted the calendar to lengthen the terms of office of their political allies and shorten the terms of their political opponents. To make matters worse, this was a local decision, so travelers going from town to town in the Roman Empire could find themselves going from year to year. After he became emperor, Julius Caesar decided that this was no way to run an empire, so in 46 BC. he instituted the Julian calendar reform. Borrowing from an Egyptian practice, the Julian calendar scrapped the lunar months in favor of months with fixed lengths by adding ten days to the calendar, resulting in months with 30 or 31 days. The one exception was February, which normally had 28 days, but henceforth would have 29 days every fourth year. The reform also made January 1 the official beginning of the new year across the Empire. The Julian calendar took effect January 1, 45 BC.

Varying Dates for Resurrection Sunday

The problem is, the tropical year, the year upon which our calendar is based, is 11 minutes short of 365.25 days.

The problem is, the tropical year, the year upon which our calendar is based, is 11 minutes short of 365.25 days. Eleven minutes may not sound like much, but over a century the difference amounts to .75 of a day. Again, .75 of a day per century doesn’t seem that large, but over the centuries, the error continues to accumulate. In AD 325, the Council of Nicea set the dates of certain Christian observances, such as Resurrection Sunday. The formula adopted for Resurrection Sunday was the first Sunday after the full moon that falls on or after March 21. March 21 was chosen because in AD 325, that was the date of the vernal equinox. However, by the 16th century, the error was 10 days. Hence, the vernal equinox was falling on March 11, not March 21. This error was causing Resurrection Sunday to occur progressively later in the spring.

To fix this problem, Pope Gregory XIII commissioned another calendar reform. The Gregorian calendar reform did two things. First, to get the calendar back to where it was in AD 325, ten days were eliminated: October 4, 1582, was immediately followed by October 15, 1582. Second, to prevent the mismatch from happening again, the algorithm for leap days was changed. The practice of treating each year divisible by four as a leap year basically continued. The exceptions were the years divisible by 100: they were not going to be leap years, except for years also divisible by 400, which would be leap years. Therefore, of the even century years since adoption of the Gregorian calendar, 1700, 1800, and 1900 were common years (years without a Leap Day), but the year 2000 was a leap year. We’ve employed the last part of the rule for leap year only once since 1582, and we won’t do it again until the year 2400. Truly, the year 2000 was one year in 400.

While Roman Catholic countries readily adopted the Gregorian calendar, Orthodox and Protestant countries did not (the Protestant Reformation was still ongoing at the time). Eventually, much of the world came to embrace the “new style” calendar. England and the American colonies changed to the Gregorian calendar in 1752, requiring the deletion of 11 days. To add to the confusion, the United Kingdom also switched observance of the new year from March 25 to January 1 that year. There were riots, with people demanding their 11 days back.

Gradual Acceptance

This slow, drawn out transition had some interesting repercussions. Russia didn’t adopt the Gregorian calendar until after the 1918 Communist Revolution requiring deletion of 13 days from their calendar. So, when was the October Revolution? In November! While it was October in Russia, it was November in much of Europe outside of Russia. Greece didn’t convert to the new style calendar until 1923. Meanwhile, the first modern Olympics were held in Athens in 1896. The Greeks sent out invitations, along with the dates of the events. Most countries understood that the dates were on the Gregorian calendar, but not the United States. The U.S. Olympic team arrived in Athens midway through the games, missing many of the events. At least for business purposes, most of the world today is on the same calendar.

Historians must be careful in how they express dates as recorded in historical documents and accounts. The convention is to express all dates prior to October 1, 1582, in the Julian calendar and to express all dates after October 1, 1582, with the Gregorian calendar. For instance, we celebrate George Washington’s birthday on February 22, 1732 (new style), but if there had been a birth certificate issued at the time, it would have read February 11, 1732 (old style). On the other hand, according to his journal, Christopher Columbus made landfall in the New World on October 12, 1492. Since this is before adoption of the Gregorian calendar, we don’t convert it to the Gregorian calendar, so October 12 is Columbus Day.

Man’s Convention Within Divine Constraints

Does any of this matter in God’s sight? Probably not.

Does any of this matter in God’s sight? Probably not. God ordained that the heavenly bodies be used to define days and years (Genesis 1:14), so one of the purposes of the heavenly bodies is to be used this way. But God left to us the details of how to do this. This is true of measures in general: God expects only that we be honest in these things (Leviticus 19:35–36; Deuteronomy 25:13–16; Proverbs 16:11; Proverbs 20:10). Establishing standards of measure, including time, is a legitimate function of government. While the day, month, and year are natural divisions of time, there is one division of time that God has ordained—the week. And the week is based on the creation account in Genesis (Genesis 1; Exodus 20:11).

There was one time when man attempted to thwart this edict. The metric system arose in the aftermath of French Revolution, and Napoleon’s conquest spread it across Europe. There are certain advantages to base-ten measures, which accounts for the popularity of the metric system around the world. But there is one metric innovation that failed to catch on. The French adopted a metric calendar, with 100 seconds per minutes, 100 minutes per hour, ten hours per day, and ten days per week. After a decade, this division of time proved to be unworkable, so the French scrapped it. The one aspect that people found most offensive was the ten-day week. It seems that even unregenerate men cannot thwart the will of God in what he has ordained.

Other Calendar Oddities

There are at least two other related issues that may be of interest to many of our readers. Due to a complicated tidal interaction between the earth and moon, the day is gradually getting longer. But don’t expect us to change our calendars anytime soon: the day is getting longer at a rate of 0.0016 seconds per century. But there is a very interesting facet to this fact. As the day gets longer, the moon’s orbit slowly spirals away. This fact places a severe upper limit to the age of the earth-moon system, indicating that the earth and moon are not billions of years old.

A second issue involves a common question that we get. Many Christians think that the year once was 360 days long, but that the year’s length catastrophically changed in the past, perhaps at the time of the flood. Nearly a decade ago, I investigated this claim in an article entitled “Was the Year Once 360 Days Long?”1 I found that there was no good evidence that the year ever was that long, so going back to creation, the day, month, and year probably had lengths similar to what they have today. Some Christians find this unsettling, because it seems to them that there not being an integral multiple of days in the month or year doesn’t fit in with their concept of a “very good” creation. But who are we to impose our concepts of “very good” on what God has done?


When do leaplings, people who were born on February 29, celebrate their birthdays during common years? Most leapers, as leaplings are sometimes known, celebrate either on February 28 or March 1 in common years. If I were a leapling, I’d probably celebrate both days.

So, enjoy your Leap Day 2020 (it is a Saturday). What is one thing people can do differently today? According to ancient tradition, it’s OK for a woman to propose to a man on Leap Day. I certainly am not one to object to tradition.


  1. Danny Faulkner, “Was the year once 360 days long?” Creation Research Society Quarterly 49, no. 2 (2012):100–108.


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