At the core, MfGames Culture is a collection of data that represents structural components of arbitrary cultures. This includes cultures and calendars that don’t have a real-world analog, such as ones created by authors and game writers.
There is also a fiddle that lets someone explore the library.
Like many of the MfGames libraries, setting up the data files can be complex but through relatively simple text files. In this case, the source are JSON files which can be easily checked into source control or served over HTTP.
These are managed by the data project and can include fiction and nonfiction cultures.
Most calendar libraries seem to focus only on the Gregorian calendar. It is hard or almost difficult to create a cross-platform that handles the Aztec calendar or the Islamic calendar, which is a lunar calendar. Even for libraries that do handle these calendars, they are frequently hard-coded to handle the system which means they cannot be easily expanded into historical or fictional ones.