Funny story about the year 2000 that does not involve Y2K: "Everyone" knows that leap years occur on year divisible by 4 ('first rule'). The second rule to them is that if a year is divisible by 100 (1800, 1900, …2000), then it is not a leap year. So a lot of folks did not have their systems/software have February 29 in the year 2000. But there is a third rule: if the year is divisible by 400 (like 1600… and 2000) it…
The 1582 document establishing the Gregorian Calendar, "Inter Gravissimas", specifically says that Y2K is leap.
https://en.wikisource.org/wiki/Translation:Inter_gravissimas
"Next, so that the equinox will no longer recede in future from the twelfth day before the Kalends of April [i.e. 21 March], we decree:
that the bissextile day every fourth year shall continue, as the custom is now, except in centurial years, although these were always bissextiles before, and we wish the year 1600 to be bissextile as well;
after that, however, the centurial years that follow shall not all be bissextiles, only every fourth centurial year shall be bissextile, thus the years 1700, 1800 and 1900 shall not be bissextile. But in the year 2000, the bissextile day shall be added in the usual way, with February containing 29 days;
and then the same order of leaving out and adding the bissextile day shall be observed in each period of 400 years ever after."