My fantasy world is one that is not based on a planet, but a plane of existence containing a nearly endless sky full of floating islands.
There are no stars. The star analog, that makes the night sky look pretty, cannot be used for navigation.
There is no sun, but rather a celestial object that acts like a sun in the sense that it has a bright period and a dark period. A day an a night.
But due to the way that this object acts, I have "in universe" names for these analogs and various units of measurement for my world.
You will be here for approximately three days.
becomes: You will be here for approximately three beats.
Did you want to stay overnight?
becomes: Did you want to stay throughdim?
I don't want to be doing this all day.
becomes: I don't want to be doing this allbright.
I have been with the institute for over thirty years.
becomes: I have been with the institute for over thirty spectra.
My worry is that overuse of my fantasy units of measurement might be annoying or even confusing to the reader, even though there is a clear in-universe reason it is called what it is.
I have the same for units of length as well, as well as some fantasy inventions to overcome the lack of a traditional sun in my world. For example, the Brightometer, a form of clock that uses lenses and diffraction of light to determine both the "time in bright" and "beat of the spectrum"; or rather how close it is to night, and how close it is to the end of the year.
A little aside. My sun analog is a sphere of unfathomable size and distance away from the surface of my world. It covers 5% of the sky directly overhead. It does not move this plane of existence does not move or rotate, so it is always in the same place in the sky. Traveling any distance in any direction on my flat surface of a world will not appear to parallax the object because of its distance and size (although it does infinitesimally).
This spherical object brightens and dims at a regular predictable interval (12ish hours). The light this object emits changes in visual color slightly over the entire spectrum a small amount continuously through red, orange, yellow, green, blue, violet, magenta, and returning to red at the end of one "year". This year length is as arbitrary as any other measurement of time. The stars pass in front of this object physically and are physically moving through space, because they are much closer to the world surface they move at considerable speed.