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:

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