Anno Domini is a Latin phrase conventionally translated into English as "in the year of the Lord." The conventional abbreviation is AD; periods after each letter are an optional matter of style, but modern usage discourages this.
AD is sometimes misinterpreted as being an abbreviation for "after death," referring specifically to the death of Jesus Christ. This causes the common confusion resulting from the general consensus is that Jesus died roughly AD 32.
In general, English usage follows Latin by placing the abbreviation before the year number for AD, but after the year number for BC, although there are several exceptions to the first part of this principle. Constructions such as "the fourth century AD" (literally "the fourth century year of the Lord") are widely accepted, but using full month-and-day with AD is not (one does not write "September 11, 2001, AD", which should be "AD September 11, 2001"). Similarly, the proper use of AD or A.D. according to its meaning is to place it before a year date, as AD 2015, "the year of the Lord, 2015", and not 2015 AD, "2015 the year of the Lord".
As the meaning of the Latin abbreviation has faded from modern pop culture memory and it became merely two letters to simply mean the years of the Christian Era, the fact that BC or B.C. is always naturally placed after a year date simply indicates to the uninformed casual person that AD or A.D. also just as naturally belongs after a year date, by a kind of parallel imitation: "2000 BC" and "2000 AD". It is a sign of creeping indifference to historical meaning among influential non-Christian people in the mainstream media, which includes the printing of business and entertainment calendars with Monday as the first day of the week and the week-end Sunday as the seventh day, no longer given first to God but leftover after working all week (which is why most people think the third day of the week is Wednesday instead of Tuesday).
BC is an English initialism meaning "Before Christ" and always is the last element. If the context does not otherwise indicate we are in such times, one is obligated to use this form. One can use it with full explicit dates, e.g., "Julius Caesar was killed March 15, 44 BC".
- Though the Anno Domini dating system was devised in 525, it was not until the eighth century that the system began to be adopted in Western Europe.