Dating the calendar
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 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.
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). The starting point is the Jewish calendar year 3761 A. (Anno Mundi) and the 754th year from the foundation of Rome. but was not in general use until ordered by the bishops at the council of Chelsea in 816 A. In the Julian calendar all centennial years were leap years (ie the years A. 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.
It is interesting to note that these terms originally applied to the date of the beginning of the year (New Year's Day).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.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.