There's a lot of fascinating discussion around Star Trek's Stardate and its seemingly universal metric calendar and times referenced as decimal units of day: How does it adjust for relativity? Why did they choose the epoch they seemingly chose? (Not that many days before Enterprise's 5-Year-Mission, and seemingly still years into Starfleet's existence.) How do you account for the various obvious real world facts that many episodes just picked numbers that sounded cool or random and didn't pay attention if they were in the right order in the season or overall timeline? (Is that relativistic problems creeping in?) They are called Stardates implying the base unit is a Day, but it is an Earth-like 24 hour day or was it something else, perhaps in a compromise with other Federation planets?

From the TNG era onward the various Writer's Bibles took an approach of 10,000 days per season to make the math easy if a script was given a Stardate in the right season and plenty of room for all the scripts in a season to have their own Stardates. If the Stardate day is a 24 hour Earth day equivalent then seasons were expected to take roughly 3 earth years. Or was that a sign that the Stardate day was about a third shorter than an Earth day and "five-year mission" did still just refer to Earth standard years?

There are all kinds of fan theories. Some weird things in canon. Some weirder things in the books and other bits of Beta and Gamma "canon".

(I did a bit too deep of a dive into Stardates recently as an offshoot of getting the idea to try to display a Stardate-like calendar and date stamps in the configurable calendar system of the Fantasy Grounds TTRPG hosting application.)