In the year of our Lord. The A.D. should precede the date and never be used with
in, since that would be
superfluous. Nor should the A.D. be used when speaking of a
century.
Wrong: He died in 1865 A.D.
Right: He died A.D. 1865.
Wrong: We live in the 20th century A.D.
Right: We live in the 20th century of the Christian Era.
From the American College Encyclopedic Dictionary, vol. II (1952)