Calendar

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.

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)

Further information can be found on the Wikipedia articles on calendars and List of Calendars.