24 hour day (it’s hard to step away from the human biorhythm too much.) 10 day week 40/4 week day month. 400 day/40 week/10 month year.
A galactic standard for trade purposes. Each planet in the galaxy would still have their own year, their own week, etc. Earth might still be on what we have today.
But in the year 3,186 AD (I don’t do the BCE/CE feel good silliness), the galaxy adopted the above standard for trade purposes. A common calendar allows traders to show up at the same location at the same time, with less likelihood of conversion mistakes between calendar systems.
Day 212 would be 12 Juni, 8428 for example. The year means nothing yet, I need to do the conversion to 400 day years on 1 Jan 3187, and bring that forward to the current time. Once I determine the day and year of the start of my first book, it’s just a matter of adding days and figuring out the day/month the event takes place.