Rodorsceaft: Pillar of Heaven
Rodorsceaft uses Spelljammer physics and cosmology. It has its own crystal sphere and the tree itself is the dominant structure within the sphere.
There are four moons: one blue, one green, one brown and one pale. The blue moon is a water body; the other three are rocky. All are size A.
The sun is, in fact, another moon: it is a fire body, also of size A, that orbits the tree’s axis of rotation.
The tree rotates on its axis approximately every 24 hours. However, the sun orbits the tree with a period of twenty days, so each “day” is a little over twenty-five Earth hours. This is relevant to spell effects; since magic is keyed to the Universal Clock which happens to coincide with the wavelength of the caesium atom, the durations of ongoing spells is adjusted accordingly.
The sun has variances in its orbit, causing seasonal changes in portions of the tree. The Sunlands, of course, see no significant change, and neither do the regions that are most branchward. The solar calendar is approximately 358 days long.
The blue moon is the most distant and has the longest period; it completes one orbit every 72 days.
The brown moon is the next one closest and has a 47-day period.
The green moon is next closest and has a 31-day period.
The pale moon is closer than the sun, and is therefore the only one that exhibits phases. It has a period of 19 days and is highly erratic; eclipses do happen.
Most calendars in use are influenced by the astronomical cycles.