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.)

In Vernor Vinge's A Deepness in the Sky, ships in the far distant future are still counting time from the Unix epoch. The common belief in the novel, though, is that it's measuring time from when humans first became space-faring by walking on the moon. But I don't recall it mentioning that there's any attempt to account for relativity; ships all keep their own local time.

Kim Stanley Robinson's Mars trilogy mentions the longer Martian day. The extra 39 minutes were represented by a 39 minute midnight clock freeze between 12:00:00 and 12:00:01. It was never mentioned how this worked across the entire planet.

Jack Campbell's Lost Fleet series (starting with Dauntless) has a lot of time involved, due to military fleets/flotillas maneuvers at light-hour distances:

Fleet A jumps into a system. Fleet B is 6 light hours distant across the system. B won't see light from A's arrival for 6 hours. A won't see B's reaction for 12 hours, less any distance covered. Ships can engage in combat at up to 0.2c relative velocity, at a maximum distance of about 1 light second.

Fleet A accelerates to 0.1c, Fleet B does the same and they approach. Time to contact is 30 hours, and the books discuss how difficult it is for humans to rest and focus on other things during that time. Our instinctive fight/flight/fawn/freeze reaction doesn't translate well to waiting 30 hours!

Fleet A and B approach, but because the ships take a few seconds to pivot/roll/yaw to maneuver, so there's a critical time period where A or B can change position and the opposing fleet won't have time to react before the fleets clash. This is used to great effect by the skilled fleet commander of the Good Guys.

The lost fleet books also discuss maneuvering fleet formations when everyone is at light-minute or multiple light second distances. There's lots of orders like "at time 15, Echo One pivot formation down 30 degrees and starboard 15 degrees". Learning to bring all your firepower to bear on the enemy at a single moment, while denying them the same opportunity, is a very difficult skill to learn in these books.

Aaand I've written way too much. And probably not done the fantastic combat in the lost fleet books any justice at all.

Famously Orson Scott Card just invented a device that explicitly ignores it.

And I can see already from the child comments you folks did not disappoint :-)