Anno Domni or A. D.

This indicates in the Year of our Lord.  It is said to have come from the Latin.

This is the time reference used by most modern documents throughout most of the world.  Some parts of the world do not consider Jesus Christ to be the same as God, but use this dating system to make things easier in order to conform to a common dating system.

Since the United States of America, the most influential country on the planet, uses this designation for dating documents it makes sense for others to use the same system of dating.

It is a rather ridiculous choice of dating since we pretty much now assume that Jesus Christ was not born in Year 1 A.D., and if we assume that Jesus Christ was actually a historical person, which seems also to be unproven.

Also we must remember that there is no year Zero since the concept of Zero was not yet developed, at least by the people of Rome and Europe.