mjhjmtu

I was writing some test cases for some convenience methods that I am updating and decided to see what would happen if I used LocalDate's isLeapYear() method on the year 0. As I understand it, year 0 never actually existed: the year before 1 AD was 1 BC. (This is based on an article I read many years ago whose source I have long forgotten.) Much to my surprise, my test indicated that the year 0 wa a leap year!

I realize that the java.time.LocalDate class implements ISO-8601 but does ISO-8601 really indicate that the year 0 existed? I'm reluctant to believe that the people who tested LocalDate would have missed this as a test case but I'm also reluctant to believe that an international standard like ISO-8601 would make such an obvious mistake.

The other possibility is that the article I read was just flat-out wrong. (Or it was correct then but was rethought later.)

This is not hugely important but I am curious to know where the mistake is: ISO-8601, Java's LocalDate class, or my understanding of how time is reckoned.

From the LocalDate Javadoc:

It is equivalent to the proleptic Gregorian calendar system, in which today's rules for leap years are applied for all time. For most applications written today, the ISO-8601 rules are entirely suitable. However, any application that makes use of historical dates, and requires them to be accurate will find the ISO-8601 approach unsuitable.

Wikipedia has more information on the proleptic Gregorian calendar. Among other things, it says:

Mathematically, it is more convenient to include a year 0 and represent earlier years as negative, for the specific purpose of facilitating the calculation of the number of years between a negative (BC) year and a positive (AD) year. This is the convention used in astronomical year numbering and in the international standard date system, ISO 8601. In these systems, the year 0 is a leap year.

Note that this has nothing to do with the AD/BC versus CE/BCE terminology: those names are only used for referring to which era of years is being used, not the calendar in use.

From Wikipedia:

...there is a year zero in astronomical year numbering (where it coincides with the Julian year 1 BC) and in ISO 8601:2004 (where it coincides with the Gregorian year 1 BC)