In English, it is found in writings as early as 1708.In Latin, the term "vulgaris aerae" (English, Vulgar Era) was used interchangeably with "Christian Era" as far back as in the 1600s.Note that if two different groups would abbreviate to the same form (e.g., both the American Psychological Association and the American Psychiatric Association abbreviate to APA), you cannot use the abbreviation in your paper—instead you must spell out the term every time to avoid ambiguity.
The Anno Domini year–numbering system was introduced by a Christian monk named Dionysius Exiguus in the 6th century.
The year count starts with year 1 in the Gregorian calendar.
Having taken considerable time solving that riddle, I realised it's always the little words and abbreviations that trip us up when learning a new language, because these little devils can turn the whole meaning of a sentence around.
In the old days they used to write full stops in between the letters, so you would at least be aware of the fact that you were dealing with an abbreviation.
But nowadays the Dutch just write them joined up, like secret codes intended to confuse poor expats who are doing their best to get a handle on this tongue-twisting language.