[quantum5]

The original French Republican Calendar determined the start of the year with the date of the autumnal equinox at the Paris Observatory. This made the calendar somewhat unpredictable, as it required a lot of astronomy to figure out the start of the year (and thus whether the previous year was a leap year). These days, it's common to see versions floating around with the same leap year rules as the Gregorian calendar instead of the original rules, leading to weird cases like having the wrong leap years compared to what happened historically.

I got nerd-sniped into making a calendar following the original rules and tried to see how far I can push it with modern astronomy, and this is the result.