This article is from the alt.usage.english FAQ, by Mark Israel email@example.com with numerous contributions by others.
"A.D." stands for "Anno Domini" = "in the year of the Lord", not
for "after the death".
Most stylebooks prescribe placing "A.D." before the year:
"Arminius died A.D. 21." WDEU calls this "the traditional and still
most frequently used styling" (the OED has citations from 1579 on);
but Collins English Dictionary says "this is no longer general
practice." Placing "A.D." after the year is, if anything, better
supported by precedents from Classical Latin (whose word order was
flexible enough that either placement would be grammatical): the
ancient Romans did not use A.D. dating, but Cicero ("Pro Flacco" I)
has "quingentesimo anno rei publicae" = "in the five-hundredth year
of the state".