|Attention, Scribe of the Lexicanum!
The Imperial Calendar is used by the Empire and by the Wasteland, a former Imperial realm. Other civilized nations have their own calendars, but seem to acknowledge and use the Imperial Calendar on occasion. It is likely that this knowledge came through commercial exchanges with these lands.
Year 0 by the calendar's reckoning was the coronation of Sigmar, as the first emperor. Events which happened before then are denoted by "-" (e.g., the rise of Nagash in Nehekhara occurred roughly in -1750 IC).