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Oh Rob, sorry. Looks like you put a fair amount of work into that...

That off-the-cuff comment wasn't meant as some sort of scholarly reference that you could track down and research. I was just adding to a story started by Paul Hirose. Sean C had mentioned that his spreadsheet had an automatic calendar transition in 1582, with no alternative for other dates. Then Paul said that he (Sean) was in good company because the JPL Horizons app does the same thing (I presume he means that it does this even now, in 2024, but I haven't checked!).

To expand on my memory of this... I came across this in one of the earliest publications of INT(x) based algorithms for calculating Julian day numbers. I can see the page in my "mind's eye" right now. Unfortunately, it's only that page, and I can't close the book to see what I'm reading :). Tricky thing memory is! The reference in question was, I believe, the Explanatory Supplement from the late 1970s (wasn't there another edition published around 1978?). And they had a neat little section way in the back explaining this seemingly miraculous new algorithm for date computations. You take the INT function of this quantity, another INT function of another quantity, maybe throw in an integer division, and, whaddaya know, the julian day number just drops right out. The time was right for the publication of this little algorithm, even in staid resources like the Explanatory Supplement (if, in fact, that decades-old bit of my memory is correct) because programmable calculators were then fairly common and home computers were beginning to take off (the Apple II computer was launched in 1977). A great many apps and tools use code directly based on those early algorithms even now... I, myself, occasionally use code derived ultimately from that article.

A feature of early versions of those algorithms was the hard-coded calendar transition in 1582. That didn't make sense then, and it certainly doesn't make sense today. Yet we still find tools today that have that date built right in (like the JPL Horizons app??). This is only a technical nuisance, of course, but it is interesting how long it has lasted. And it's not terribly hard to find other tools that force the calendar change to the singular date in October 1582. This seems to be especially common in older astronomy tools. It's hard to imagine what people were thinking... I suspect that were simply adhering to rules and copying code that eventually may be traced back to some of those resources from the early days of home computers in the late 1970s.

Frank Reed