On one of the last test runs I will do, it seems, the in game sunrise hour happened at calculated sunrise hour for a position 258-230 km west of my position, ingame sunset happened at calculated hour for a position 274-302 km east of my position. This makes no sense at all. No time zone or mismatching ship/local/GMT time can explain that as far as I'm aware.

I will do one last attempt, going to various positions at a few constant latitudes to see if the offsets show any pattern, then at a few constant longitudes. Unless I can find that pattern, I will say the game runs a sun cycle of its own creation.