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The calendar has an interesting history, and has been shaped by both political ideals and a quest for greater accuracy. Recorded history is not precise on all dating methods in use, let alone the exact dates that every change occurred, but I have pieced together an account of many key events. The method for calculating Easter date also mirrors calendar changes, so I have included that also. Many thanks must go to Ron Mallen for his tireless, meticulous and scientific process in researching this history.
There is a chart below that graphically shows the key events shaping the calendar. This history starts on the "Kalends of March" or March 1st with the introduction of the Roman calendar in the year 1 AUC (AUC stands for Ab Urbe Condita, meaning "from the foundation of Rome"). 1 AUC is the same as 753 BC in the Julian calendar. The Roman AUC calendar was enforced (with capital punishment for non-compliance) throughout the powerful Roman Empire of the time.
It started as a year of 10 lunar months, and soon changed to a lunar year of 12 months. Other enhancements were made to change to solar years, with patchy attempts to add additional days to maintain alignment of seasons.
The Julian calendar was introduced in 709 AUC (or 45 BC) and was quite similar to our current Gregorian calendar. It had 12 months, and attempted to measure solar years by using occasional 366-day years.
Of course, at the time of Jesus' life, years were not called BC and AD; Roman AUC years were used. It was not until 532 AD that the Pope, with significant influence, replaced Julian AUC years with Julian AD years.
It is a common misconception that AD years were set so that Jesus was born in 1 AD. This is not correct. 1 AD was set to meet two criteria:
With recorded history at the time (in 532 AD), it was known that Jesus was alive on January 1st, 3BC. So 1 AD was set to the next year that allowed 366-day years to occur in AD years exactly divisible by 4. It is now apparent that Christ was born during 5 BC, and was therefore alive on January 1st, 4 BC, but this was not certain in 532 AD.
Even though 366-day years were nominally set to occur in 4 AD and every 4th year afterwards, it happened that the 366-day year was skipped in 4 AD as the final adjustment for having too many 366-days in previous years. So 366-day years resumed in 8 AD and every 4 years thereafter.
The Gregorian calendar was introduced over a period of many years. Italy was the first to use it in 1582, while Greece introduced it in 1923! This calendar corrects accumulated inaccuracies with the Julian calendar by having slightly less leap years. We still have leap years every 4 years except that century years (ending with "00") are leap years if they're evenly divisible by 400. This means that only 1 in 4 century years is a leap year (ie 1600, 2000, 2400, etc). This calendar is so accurate that a further adjustment will not be required until 4100 or soon after.
Here are some clarifications to common misconceptions about calendars:
The following excellent and accurate article is reprinted from the Australian EsE-2c (Easy To See) Calendar 1998. Unfortunately, no author or source is credited.
The calendar as we know it has evolved from a Roman calendar established by Romulus, consisting of a year of 304 days divided into 10 months, commencing with March. This was modified by Numa, who added two extra months, January and February, making a year consist of 12 months of 30 and 29 days alternately plus one extra day and thus a year of 355 days. This calendar required the use of an Intercalary month of 22 or 23 days in alternate years.
In the year 46 B.C. Julius Caesar asked for the help of the Egyptian astronomer Sosigenes, as he had found that the calendar had fallen into some confusion. This led to the adoption of the Julian calendar in 45 B.C. In fact, the year 46 B.C. was made to consist of 445 days to adjust for earlier faults and is known as the "Year of Confusion".
In the Christian system, years are distinguished by numbers before or after the Incarnation, being denoted by the letters B.C. (Before Christ) and A.D. (Anno Domini). The starting point is the Jewish calendar year 3761 A.M. (Anno Mundi) and the 754th year from the foundation of Rome. This system is said to have been introduced into England by St. Augustine about 596 A.D. but was not in general use until ordered by the bishops at the council of Chelsea in 816 A.D.
In the Julian calendar all centennial years were leap years (ie the years A.D. 1200, 1300, 1400 etc.) and for this reason towards the end of the 16th century there were found to be a difference of 10 days between the Tropical and calendar years. This was corrected in 1582 when Pope Gregory ordained that October 4th would be followed by October 15th, making the 10 day correction, and that only every fourth centennial year should be a Leap Year. This is known as the Gregorian calendar and is the one we now use.
It was adopted by Italy, France and Portugal in 1582 and other countries made the correction at various dates up to as recently as 1923. The change from the Julian to the Gregorian calendar took place in England and her dominions in 1752, when the correction was made by the omission of eleven days; Wednesday, September 2nd being followed by Thursday, September 14th.
The Julian and Gregorian calendars are sometimes referred to as the Old Style and New Style calendars. It is interesting to note that these terms originally applied to the date of the beginning of the year (New Year's Day). In the Old Style this was March 25th and was changed to January 1st (New Style) in England in 1752 when changing from the Julian to Gregorian calendar. New Years Day was changed to January 1st in Scotland in 1600.
The Equinoctial or Tropical Year is the time that the Earth takes to revolve around the Sun from one Spring Equinox to another. This is approximately 365.24219 mean solar days, or 365 days, 5 hours, 48 minutes and just over 45 seconds. The Equinox is the point where the Sun crosses the Equator making day and night equal.
The Calendar Year is 365 days except if the year number is divisible by four evenly, this being a Leap year of 366 days. The last year of a century is not a leap year unless its number is evenly divisible by 400. For example, 1800 and 1900 were not Leap Years, while 2000 is a Leap Year.