Who, exactly, declared the first year, AD and who agreed to it? Wat did they use before Christ supposedly died?
The system of BC and AD was developed by a Greek monk living in Rome in the land fifth/early sixth century named Dionysus Exiguus.
The Venerable Bede's Ecclesiastical History is the first major (I don't know if there is some lesser known work that adopted it earlier) work to use the system. Bede also uses the old Roman dating system in his text. That is based on the consul of the year. By the later Empire, there was a new consul every month, and often times the emperors themselves served as consul. This meant that the naming of the year went to the first non-emperor consul of that year. Often times Principate era texts also use the regnal year to denote the date, i.e. in the x year of y emperor, during the consulship z.
It should be noted that these are only two dating systems among a myriad of others. For example, the Hebrew calendarcalculates dates in terms of the creation of the world according to the Penteteuch. For example, we are now in the year 5774. The Muslim calendar centers history around the hijra, or the migration of Muhammed and his followers to Medina. Thus, the birth of Muhammed occured in the year 53 BH and we are currently living in the year 1435.
I don't know the system devised by Dionysus Exiguus came to be so widespread and supplanted other systems used throughout the world.
I'm not sure if you are aware of it or were looking for a text-based answer but one of the topics of AskHistorians podcast has been "What year is it?" which answers and discusses everything you have asked and more. It can be found here : http://www.reddit.com/r/AskHistorians/comments/20pftl/askhistorians_podcast_episode_006_discussion/
hi! you'll find lots of relevant info on years, and BC/AD in the FAQ (link on sidebar):