Interesting. In one place the author argues: 0 is missing, but we already know...
The is no year 0, it goes 1 BC, 1 AD. So testing whether 0 is a leap year is moot.
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Not true if you use astronomical year numbering: https://en.m.wikipedia.org/wiki/Astronomical_year_numbering
Which is arguably the right thing to do outside of specific domains (such as history) in which BCE is entrenched
If your software really has to display years in BCE, I think the cleanest way is store it as astronomical year numbering internally, then convert to CE/BCE on output
Without that design constraint, testing for leap years becomes locale-dependent and very complex indeed.
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."
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.
At work we had discussions what date format to use in our product. It's for trained users only (but not IT people), English UI only, but used on several continents. Our regulatory expert propsed ISO8601. I did not agree, because that is not used anywhere in daily life except by 8 millions Swedes. I voted 15-Apr-2025 is much less prone to human error. (None of us "won". Different formats in different places still...)
However, in general, I think proleptic Gregorian is simpler. But in astronomy do what the astronomers do. And in history, dates between 1582 and 1923 (inclusive), you really need to explicitly mark the date as Gregorian or Julian, or have contextual information (such as the country) to determine which one to use.
1923 because that was when Greece switched from Julian to Gregorian, the last country to officially do so. Although various other countries in the Middle East and Asia adopted the Gregorian calendar more recently than 1923 - e.g. Saudi Arabia switched from the Islamic calendar to the Gregorian for most commercial purposes in 2016, and for most government purposes in 2023 - those later adoptions aren’t relevant to Julian-Gregorian cutover since they weren’t moving from Julian to Gregorian, they were moving from something non-Western to Gregorian
Large chunks of the Eastern Orthodox Church still use the Julian calendar for religious purposes; other parts theoretically use a calendar called “Revised Julian” which is identical to Gregorian until 2800 and different thereafter - although I wonder if humanity (and those churches) are still around in 2800, will they actually deviate from Gregorian at that point, or will they decide not to after all, or forget that they were officially supposed to
>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.
Does it matter? MM-DD-YYYY is used in America and makes DD-MM-YYYY ambiguous, but as far as I know nobody uses YYYY-DD-MM, so ISO8601 should be perfectly fine, especially if users are trained. Besides, if you're not used to it, starting with the year forces you to think, which is desirable if you want to avoid human error.
However, due to a mistranslation the Roman pontifices got it wrong at the introduction of the Julian calendar. The Romans counted inclusively, which means: counting with both the start and end included. (That is why Christians say in a literal translation from Latin that Jesus has risen on the third day, even though he died on a Friday and is said to have risen two days later, on the next Sunday.)
In the first years of the Julian calendar, the Roman pontifices inserted a leap day “every fourth year”, which in their way of counting means: every 3 years. Authors differ on exactly which years were leap years. The error got corrected under Augustus by skipping a few leap years and then following the “every 4 years” rule since either AD 4 or AD 8. See the explanation and the table in https://en.wikipedia.org/wiki/Julian_calendar#Leap_year_erro...
Also note that at the time, years were mostly identified by the names of the consuls rather than by a number. Historians might use numbers, counting from when they thought Rome was founded (Ab urbe condita), but of course they differed among each other on when that was. The chronology by Atticus and Varro, which placed the founding of the city on 21 April 753 BC in the proleptic Julian calendar, was not the only one.