The Lotharian Calendar (L.C. and P.C.) was adopted after the coronation of Varian Wrynn, and, due to Stormwind's prominence among the nations of the Grand Alliance, would become the basic calendar for many of those under it. Its Year Zero is the opening of the Dark Portal and the start of the First War. It is named in honour of Anduin Lothar.
For ease of use, and to avoid writing dates in negative, the custom eventually came about of writing dates before 0 L.C. as being 'P.C.', or 'Pre-Calendar' (the commonly supposed 'Pre-Calamity' is a neologism). Thus, the Dark Portal reopened in January 29 L.C., the Invasion of Northrend began in the winter of 30 L.C., while the independence of Stormwind was in 592 P.C. An adapted companion of this calendar also utilizes the designations 'A.P.' and 'B.P', for 'Before' and 'After Portal' respectively.
The present year in the Lotharian Calendar is 35 L.C.