B.C. or B.C.E.? Many people use the abbreviations B.C. and A.D. with a year (for example, A.D. 2012). B.C. refers to “Before Christ,” and the initials, A.D., stand for Anno Domini, which is Latin for “In the year of our Lord.” This system was devised by a monk in the year 525.
A more recent system uses B.C.E. which stands for “Before the Common Era” and C.E. for “Common Era.” This newer system is now widely used as a way of expressing the same periods as B.C. and A.D., but without the Christian reference. According to this system, we count time backwards Before the Common Era (B.C.E.) and forwards in the Common Era (C.E.).
Often dates will be preceded with a “c.” or a “ca.” These are abbreviations of the Latin word “circa” which means around, or approximately. We use this before a date to indicate that we do not know exactly when something happened, so c. 400 B.C.E. means approximately 400 years Before the Common Era.
Our calendar is called the Gregorian calendar and was instituted by Pope Gregory XIII in 1582. There are many other calendars. Quite a few societies have used calendars linked to the years their kings ruled. And there are numerous calendars, beyond the Gregorian calendar, that are still in use today. For example, 2012 equates to 1434/35 in the Islamic calendar and 5772-73 in the Jewish calendar (both are lunar, based on the cycles of the moon).