That's a fun easter egg in the TZ database. I'd never heard of it, but how often do you need to calculate TZ data that old. Hopefully we'll be done with time zone's completely before the theory is outdated.

We'll never be done with timezones as long as we have biological ties to the Sun.

Even if you mandate that wallclocks agreed everywhere on Earth, everyone would need a replacement to know when someone is likely to respond: Is it too late to call Grandpa? and: When should I schedule this business meeting?

This is really neat!

Separately, I'm a little skeptical of the tzdb's endeavor of even thinking about pre-Unix-epoch stuff. The bug-to-utility ratio of that stuff doesn't seem to be there. `zic` is where most of the ugly gnarliness of tzdb lies, and I sometimes feel that it would have been better if `zic` weren't an artifact others could depend on.

There are plenty of people alive today with negative birth timestamps, with many services wanting to calculate the proper local day and time each year to send them annoying pseudo-personalized birthday messages.

Or, for something a bit more agreeable: genealogy (esp. the bioinformatics queries in genetic genealogy) needs a good, high-resolution timeline to normalize events onto. Actually, the non-genetic kind of geneaology, solves a lot of vagueries in lineage as constraint-based puzzles over (local!) dates, with razor-thin margins that would be messed up without correct timezone-based calendar conversion — constraints like "two people couldn't have been related as mother and child, if the mother died at least two days before the child was born."

Honestly, that seems quite practical — it's a good way to head off "angels dancing on the head of a pin"-style arguments between contributors.

Another way to put the bug outcome is: "Instants before the Big Bang are out of scope of this library, so if an algorithm produces incorrect values, but only before the Big Bang, the algorithm is acceptable and does not need to be improved/replaced."

There used to be a long manpage about linux/unix `date` and what dates meant in the past - with examples of 15 century and so. Talking about some king who liked some week/month and decreed it must be repeated, and another who disliked some week and scratched it off the calendar.. It was very eye-opening reading.

But i can not find it anymore. Anyone has an idea ? Is it gone completely?

man cal:

       Two different calendar systems are used, Gregorian and Julian.
       These are nearly identical systems with Gregorian making a small
       adjustment to the frequency of leap years; this facilitates
       improved synchronization with solar events like the equinoxes.
       The Gregorian calendar reform was introduced in 1582, but its
       adoption continued up to 1923. By default cal uses the adoption
       date of 3 Sept 1752. From that date forward the Gregorian
       calendar is displayed; previous dates use the Julian calendar
       system. 11 days were removed at the time of adoption to bring the
       calendar in sync with solar events. So Sept 1752 has a mix of
       Julian and Gregorian dates by which the 2nd is followed by the
       14th (the 3rd through the 13th are absent).

https://man7.org/linux/man-pages/man1/cal.1.html

no, not that. It was like 5-10 full pages, talking about history, mostly western european but not only..