Gregorian and Julian Calendars

The Gregorian calendar is the calendar system that is used today. It is a revision of the Julian calendar that preceded it.

Until 1582 AD, western nations measured time using the Julian calendar. This calendar was authorized by Julius Caesar in 46 BC. It is based on the assumption that a year is 365.25 days. Every fourth year is declared a leap year when an extra day is added to February.

Since a true year is about 365.2422 days, the accumulated error adds a full day about every 128 years. In 1582, Pope Gregory XIII authorized the Gregorian calendar to compensate for much of the error in the Julian calendar. The Gregorian calendar is the same as the Julian calendar, except years evenly divisible by 100 are not leap years (e.g., 1900) unless they are also evenly divisible by 400 (e.g., 2000). The Gregorian calendar uses the assumption that the true year is 365.2425 days. This calendar still has an error factor of three days every 10,000 years, and eventually a correction will have to be made.

When the Gregorian calendar was authorized, a correction was made to attempt to bring the current date in line with dates in the year 1 in the Julian calendar. The correction was miscalculated by two days and only ten days were dropped instead of twelve. Pope Gregory decreed that the day following October 4, 1582, should be called October 15, 1582. To complicate matters, not everyone switched at once. The following table shows when various nations converted:

Be suspicious of any dates from a given country near the time when the switch was made; they may be in either system. Julian dates should be marked with O.S., for old style, but books are lax on including this notation, particularly discussing dates around the Spanish Conquest where they are needed most.

Note that neither the Gregorian nor Julian calendars have a year zero; the calendars jump from 1 BC to 1 AD. Because of this, the years 1 BC, 5 BC, etc., are leap years.