When did people start explicitly referring to the year they were in (e.g. 158 AD)? I imagine it was way after the year 1AD, of course. How did they calculate it, and when did they do it?

Mentioning previous answers is not intended to discourage more discussion -- follow-on questions, information, or debate is welcome.

There's an FAQ section, Calendars, at https://www.reddit.com/r/AskHistorians/wiki/language#wiki_calendars . "At what point did society begin referring to the year in modern terms (2012 etc)" is an article about the spread of AD/BC dating, but they also talk about other calendar systems. The section also points to AskHistorians podcast 6, "How did we decide what year it is?", http://askhistorians.libsyn.com/askhistorians-podcast-episode-006 , and six other discussions.