The Creation of Leap Year and Its Effects on Genealogy

This Saturday is February 29th, a relatively uncommon occurrence as days go. It’s a leap day in a leap year. Unless you’re like Dinah Shore, Tony Robbins, or Ja Rule, all of whom share a birthday that day, you may not even notice. For the rest of us, it’s a subtle reminder that calendars change. For genealogists in particular, it reminds us that subtle changes in calendars require our attention in researching key times and places.

Our calendar, and the most commonly used calendar in the world, is the Gregorian calendar, named after Pope Gregory XIII who commissioned it in 1582. In order to best understand the Gregorian calendar and its significance, however, we need to first understand its predecessor, the Julian calendar, and its accomplishments.

Among the world’s earliest calendars, many were lunar, which determined the length of year by tracking the position of the Moon. Lunar calendars differed from solar calendars, which determined the length of a year by tracking the position of the Earth relative to the Sun. The difference between a lunar year measured by Moon phases and a solar year measured by Earth’s orbit around the Sun is about 11 minutes.

Later calendars attempted to account for the 11-minute difference and its accumulated effects by periodically adjusting themselves to synchronize the two measurements. In these cases, the occasional months added are referred to as intercalary or interstitial months. The calendars that employed these methods are referred to as lunisolar.

Prior to the reign of Julius Caesar, the Roman calendar was lunisolar, but was also complicated at best. Early Roman calendars consisted of ten months with a vaguely defined winter period. Over time, this winter period was replaced by the two additional months of January and February. The calendar, however, was still problematic as it had become completely unconnected to the solar year. One solution was to develop a thirteenth, intercalary month of 23 days called Mercedonius, which was inserted within the month of February. That solution, however, was politicized as Roman consuls determined when and for how long to hold Mercedonius, adjusting the calendar to fit their political needs.

To standardize the Roman calendar, Julius Caesar turned to the Egyptian calendar, which consisted of 365 days. To keep the calendar attached to the Sun (a solar year is 365.2421 days), Egyptian astronomers inserted an intercalary month from time to time. Ceasar and the Egyptian philosopher Sosigenes of Alexandria combined forces to adapt the calendar slightly. Instead of depending on astronomers to determine when an intercalary month should be inserted, they simply added one day every fourth year, which was added to the end of February. With these parameters the Julian calendar was introduced by edict and began 1 January 45 BCE.

For all its improvements, the Julian calendar was still slightly longer than the solar year. Over the centuries the slight inaccuracy compounded until the calendar was ten days behind by the beginning of the 16th century. Pope Gregory XIII sought to improve the accuracy by making two key adjustments. First, he updated the calendar by removing ten days from October 1582. Second, he determined that leap years (generally every four years) should not be counted in centennial years that are not divisible by four. So the years 1700, 1800, and 1900 were not leap years, but 2000 was.

As with the Julian calendar, the Gregorian calendar was adopted over time by most of the Western world. But the adoption wasn’t automatic or universal. Protestant German states, for example, didn’t adopt it until 1700, nor did Great Britain until 1752.

A key feature of the Gregorian calendar was that the year began on 1 January, just as the original Julian calendar had. While most of Europe and its colonies elsewhere adopted the calendar, the start of the year varied by location and period. In England, the new year was either 25 March or 25 December until shortly after the Norman Conquest in 1066. Between 1087 and 1155, it was 1 January. Then from 1155 until 1752, England’s year began 25 March. Finally, from 1752 until the present, the Gregorian calendar was adopted, and New Year’s Day was once again January 1st. Each European country has a different history for its New Year’s Day.

Genealogists researching in England, its territories, or former territories don’t need special knowledge of calendrical history if they’re researching after 1752. But researchers of colonial America, for example, do need to be aware of some of these details. English colonies in the Americas used 25 March as the beginning of the calendar year until 1752. That means that dates written on documents need to be considered and recorded thoughtfully.

A twin born in colonial America late on the evening of 24 March 1741 may be followed just a few minutes later by the birth of a sibling in the early morning hours of 25 March 1742. Unless one is familiar with the Gregorian calendar and its adoption by the British, one might think they were born a year apart. When we record these two births, we clarify the situation by adding a bracketed slash and the Gregorian year (e.g. 24 March 1741[/2] and 25 March 1742). The brackets signal to the reader the fact that the researcher has added the clarification based on her knowledge of the calendar in use in the time and place she’s researching.

If you’re researching people of the English colonies, the details given here should suffice in preparing you to understand dates as they’re written. But if you’re researching Spanish, French, or Dutch colonies, for instance, you’ll need to conduct additional research to understand when the Gregorian calendar was adopted locally and how dates were recorded.

For more details about the Julian and Gregorian calendars, New Year’s Days, and other world calendars, see, for example, “General Chronology,” New Advent ( : accessed 27 February 2020), especially the sections titled “Beginning of the Year,” “The Gregorian Reform,” and “Julian Period.” For more details about Leap Day, see, for example, Stephen Wood, “5 Things You May Not Know About Leap Day,” History ( : accessed 26 February 2020).

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