Is there a proper term for the origin on the timeline from which we count years? For example, we often have years in BC/AD or BCE/CE - what is this point of change from one to the other called. Origin, axis, omphalos, or what?
At the time when the so-called Christian era was introduced, the mathematical concept of zero was not known in Europe. Thus, the year before AD 1 is called 1 BC. However, in modern astronomical and mathematical usage “1 BC” is called the year “0”, “2 BC” is called “-1”, “3 BC” is called “-2”, etc. If you follow this convention you can carry out normal mathematical operations with year numbers. The starting point of the Christian calendar is 1 January AD 1; this is called the “epoch” of the Christian era Julian style. The equivalent date in the Gregorian calendar is 30 December year 0 (alias 1 BC). The epoch of the Gregorian calendar is thus the equivalent of 3 January year 1 (AD 1) Julian.
In chronology there is no year 0. The first year is year 1. The year before that is year -1. The origin of a time system is called its epoch.