It started as a year of 10 lunar months, and soon changed to a lunar year of 12 months.
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.
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. 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.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.