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" are widely accepted, but using full month-and-day with AD is not (one does not write "September 11, 2001, AD").
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.