All this data is potentially out of date, and should be taken with a truckload of salt
The YC calendar is the universal calendar of New Eden. It was created under the CONCORD treaties that govern universal time. YC stands for "Yoiul Conference", the conference between the five empires of New Eden: (Amarr, Minmatar, Caldari, Gallente and Jove) that led to the forming of CONCORD, held on the Jove cruiser Yoiul.
Year YC0 of the YC calendar corresponds to the year of the conference and respective treaties (23236 A.D.) It utilizes a reconstructed calendar used by the ancient settlers of the cluster.