Calendar

The shift between the Julian and Gregorian calendars is one of the most fascinating—and confusing—quirks of recorded history. It is the reason why some historical dates seem to contradict each other depending on who was writing them down.

Here is a straightforward breakdown of how both calendars work, why the switch happened, and how it affects the dates we read about today.

Introduced by Julius Caesar in 45 BC, the Julian calendar was a massive improvement over the chaotic lunar calendars that came before it.

• The Math: It was based on the premise that a solar year (the time it takes Earth to orbit the Sun) is exactly 365.25 days long.
• The Leap Year: To account for that extra quarter-day, the calendar added one extra day to the month of February every four years.
• The Flaw: The actual solar year is roughly 365.2422 days long. The Julian calendar overestimated the length of a year by about 11 minutes.

While 11 minutes doesn’t sound like much, it adds up to an extra day roughly every 128 years. By the 1500s, the calendar had drifted completely out of sync with the solar seasons. The vernal (spring) equinox, which the Christian church used to calculate the date of Easter, was occurring a full 10 days earlier than it was supposed to.

To fix the drift, Pope Gregory XIII introduced the Gregorian calendar in October 1582. This is the internationally accepted civil calendar we still use today.

To correct the errors of the Julian system, the Gregorian reform did two major things:

1. The Great Leap Forward (Dropping Days) To immediately realign the calendar with the seasons, 10 days were quite literally deleted from existence. In countries that adopted the calendar immediately (like Italy, Spain, and France), Thursday, October 4, 1582, was followed directly by Friday, October 15, 1582.

2. The New Leap Year Rule To prevent the drift from happening again, the leap year math was slightly adjusted. A leap year still happens every four years, with one exception for “century years” (years ending in 00):

• A century year is not a leap year unless it is divisible by 400.
• Therefore, 1700, 1800, and 1900 were not leap years, but 2000 was. This tiny adjustment keeps our calendar incredibly accurate.

The transition from the Julian to the Gregorian calendar did not happen overnight. Because it was introduced by a Catholic Pope during the Protestant Reformation, many non-Catholic countries refused to adopt it for centuries. This caused massive dating discrepancies across the globe.

• Great Britain and its Colonies (including early America): Did not switch until 1752. By then, they had to drop 11 days to catch up. Wednesday, September 2, 1752, was followed by Thursday, September 14.
• Russia: Did not switch until 1918. By that time, the Julian calendar was 13 days behind. This is why the famous Russian “October Revolution” of 1917 is celebrated in November on the modern Gregorian calendar.

Old Style (O.S.) vs. New Style (N.S.) When looking at historical documents—especially genealogies or letters from the 1500s through the 1700s—you will often see dates marked as O.S. (Julian) or N.S. (Gregorian).

The George Washington Example: > George Washington was born under the Julian calendar. His original birth date was recorded as February 11, 1731 (O.S.). When the British Empire finally adopted the Gregorian calendar in 1752, his birthday officially shifted by 11 days. Furthermore, the British also officially moved the start of the New Year from March 25 to January 1. Consequently, Washington’s birthday is now universally recognised as February 22, 1732 (N.S.).