solar calendars:
the Roman calendar became the Julian calendar became the Gregorian calendar (composed of 97 leap years every 400 years)
lunar calendars:
such as the Chinese calendar (not, in fact, the official calendar of China), the Mayan calendar (with its five “nameless days”), and the Hebrew calendar (which adds an extra month every 19 years)
fun words: intercalary, Metonic, computes, and epagomenal
the lesson: time is oh-so relative