In OTL there are only a few calendars in general use: the Gregorian/Western calendar is the most widely used, with others tending to be more local, and only occasionally giving rise to confusion (there being a few examples of ambiguities arising when the Julian calendar was more widely used). In althistory timelines a calendar other than the Gregorian could be the norm, or several calendars could be in use in the same locality. For timelines in which there are a number of inhabited planets, a more complex structure with correlations would have to be developed - possibly planetary/star-system calendars for purely local use and one for use between systems.
Here is a list of some calendars discovered throughout the alternate realities:
- Calendars (Superpowers)
- Celtic Calendar (Celtic Rules)
- Italian Republican Calendar (Principia Moderni IV Map Game)
- Modern Era Calendar (Day of Glory)