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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The is no year 0, it goes 1 BC, 1 AD. So testing whether 0 is a leap year is moot.
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
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