Eurentus follows a thirty month calendar that was devised at the end of the Eurentus Civil War. The months are simply the numbers one through thirty in Spanish. Every other year there are thirty-one months in the year.
P.M.E. stands for Pre Modern Era, which counts down until the end of the Eurentus Civil War and start of the calendar.
P.C.W. (also marked as just the date) stands for Post Civil War. It counts upwards past the date that the calendar was started.