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.