Specifically wondering about ancient Roman and Greek historians, as I imagine different societies had different ideas about time.
You might want to look at http://www.reddit.com/r/AskHistorians/comments/2efg4k/bc_ad_what_did_they_use_at_the_time/
The Greeks tended refer to years using annual public offices - "when so-and-so was archon", for example. They might also count the years from the start or end of a certain war.
Similarly, the Romans gave the names of the two consuls for the year to which they were referring. Alternatively, they counted the years ab urbe condita, from the founding of the city.
hi! you'll find a few posts on the development of the Gregorian calendar (i.e. the one we're using now) and other dating systems in the FAQ (link on sidebar):