We chose to use the year zero in the astronomical year
numbering as a reference point, as it is also the reference
used by Calendar.ISO, the default calendar used by Elixir,
It is also the same epoch used by Erlang's :calendar module.
Note we chose to not call it gregorian epoch due to the confusion
between January 1 AC 1 and 0000-01-01. The latter actually maps
to January 1 BC 1.
We also moved away from Calendar.Julian, which could have been
based on Calendrical Calculations, and chose Calendar.Holocene,
which adds exactly 10,000 years to Calendar.ISO and as such has
a very straigh-forward implementation.
More information:
+ https://en.wikipedia.org/wiki/Year_zero
+ https://en.wikipedia.org/wiki/Holocene_calendar