Just do what all other writers have done and choose a starting point and then increment the numbers a bit every time. That horse has not only bolted, it ran past the horizon, had a fulfilling family life, and died of old age.

https://en.wikipedia.org/wiki/Stardate

“This makes it impossible to convert all stardates into equivalent calendar dates, especially since stardates were originally intended to avoid specifying exactly when Star Trek takes place.”

This sent me down a rabbit hole, and I'll freely admit I only went as far down as I could still see the entrance behind me, because man, that is a rickety warren.

IRL there would be mind boggling complexities of calculating an objective system for marking time, when you have starships zipping about at faster than light speed. The subjective time of those crews would diverge from standard observed time, and between ships traveling in different directions, at different warp speeds, and for varying stretches of (subjective?) time.

The above is my dumbed-down understanding of time dilation. But the real real life explanation is that Gene Roddenberry and writer Samuel Peeples got tipsy and drew up their own galactic map with travel itineraries (presumably on napkins and backs of coasters 🙂):

"For the starship captain's log entry narrations, Roddenberry wanted to devise a futuristic measurement of time reference. He called (Sam) Peeples (whom Roddenberry had contacted early on for help in learning about science fiction, a subject he knew nothing about; it was Peeples who wrote "Where No Man Has Gone Before," the pilot that sold ST). The two men had a few drinks while brainstorming, and soon began chuckling over their imaginative 'stardate' computations. 'We tried to set up a system that would be unidentified unless you knew how we did it,' Peeples says."

"They marked off sections on a pictorial depiction of the known universe and extrapolated how much earth time would elapse when traveling between given points, taking into account that the Enterprise's warp engines would be violating Einstein's theory that nothing could exceed the speed of light. They concluded that the 'time continuum' would therefore vary from place to place, and that earth time may actually be lost in travel. 'So the stardate on Earth would be one thing, but the stardate on Alpha Centauri would be different,' Peeples says. 'We thought this was hilarious, because everyone would say, "How come this date is before that date when this show is after that show?" The answer was because you were in a different sector of the universe.'"

— Joel Engel, Gene Roddenberry: The Myth and the Man Behind Star Trek, cited in the Memory Alpha entry for "Stardate".

During S1 of TNG is when it was first standardized. Beginning with 40xxx (2364), it increased to 41xxx (2365) and so on for every calendar year that TNG, DS9, and VOY were in production, including the TNG films. Star Trek Lower Decks, Prodigy, and Picard also follows the same format consistently. If there’s any logic beyond that, I don’t know of it.

I’ve shared this here before. It’s the most extensive analysis of stardates I’ve encountered: atavachron.wikidot.com

I like to think it's kinda like Semantic Versioning in programming