The traditional Japanese calendar? If I ever work on that Ralts-line conlang I often mention, they'll use that.
...I'm not sure what this is supposed to mean exactly; Japan used Chinese-derived systems until consistent contact with the west and then adopted the Gregorian calendar plus 660 years (kōki/kigen). Did you have some specific system in mind?
I expect the system will be sophisticated enough to support specifying the whole Gregorian calendar, as well as intercalation (blame pthag), so there shouldn't be an issue with kōki.
I will, however, make a point of supporting named years and eras, since that seems to be important.
The former, actually, month lengths that literally vary with the phases of the moon and all.
“Alice locks her signed copy of the book in the chest with her lock. She sends the chest to Bob. He slaps his lock on it, and sends the chest back to Alice. She correctly interprets this as a big middle finger from Bob, and takes a chainsaw to open up the chest and get her damn book back since he obviously doesn’t want it.”