[rcoveson]

No, old calendar systems are one indexed. In those system, there is literally no year zero; the first year is year one. This leads to crazy things like year 100 being part of the "first century" and year 101 being part of the "second century".

That is not the case with age or birthdays which are, thankfully, zero indexed. The first year of human life is age=0, birthdays=0.

[samatman]

So that we don't create more falsehoods programmers believe about dates, ISO has defined decades and centuries as starting with 0, not 1.

So as far as your job is concerned, that's when they start. Hope that helps.

[lupire]

It's not "crazy". No one care about which century X00 is. The term "century" doesn't have enough sig figs. X00 is "the turn of the century".

[rcoveson]

If you don't care then sure, it's not crazy, but if you did care then believe me, it is crazy. Crazy enough that astronomers[0] and software engineers[1] rebelled against the historian's practice and renamed the years preceding 1 AD in the proleptic gregorian calendar year 0, year -1, year -2, et cetera. A major benefit of this is it allows the leap year pattern to stay consistent and the rule to remain legibile for all years, going back before 1 AD. It's also nice because it lets us say "the 90's were the last ten years of the 20th century" and be correct.

0. https://en.wikipedia.org/wiki/Astronomical_year_numbering

1. https://docs.oracle.com/javase/8/docs/api/java/time/temporal...