[nmehner]

https://listverse.com/2019/05/19/10-bizarre-calendar-fixes-t...

Everything before the introduction of the gregorian calendar is moot:

"In 1582, the pope suggested that the whole of Europe skip ten days to be in sync with the new calendar. Several religious European kingdoms obeyed and jumped from October 4 to October 15."

So you cannot use any date recorded before that time for calculations.

And before that it gets even more random:

"The priests’ observations of the lunar cycles were not accurate. They also deliberately avoided leap years over superstitions. Things got worse when they started receiving bribes to declare a year longer or shorter than necessary. Some years were so long that an extra month called Intercalaris or Mercedonius was added."

[usr1106]

Before 1582 the rule is just simpler. If it is divisible by 4 it's a leap year. So the difference is relevant for years 300, 500, 600, 700, 900 etc. For ranges spanning those years the Gregorian algorithm would result in results not matching reality.

When the Julian calendar was really adopted I don't know. Certainly not 0001-01-01. And of course it varies by country like Gregorian.

[pbhjpbhj]

From Wikipedia:

>The Julian calendar was proposed in 46 BC by (and takes its name from) Julius Caesar, as a reform of the earlier Roman calendar, which was largely a lunisolar one.[2] It took effect on 1 January 45 BC, by his edict.

Not knowing the year seems unhinged somehow.