We all know how inclusive counting affects the reckoning of days. "Two days ago" is nudius tertius, and so on.
Is this method also used in reckoning months and years? Is "two years ago" translated as tribus annis abhinc? If so, then does it follow that "one year ago" must use some other adjective like anno proximo?
I would say
abhinc biennium for "two years ago". The adverb
abhinc can take either an ablative of degree of difference or an accusative of extent of time, but I believe the latter is more common. I'm really not sure how to say "one year ago". Analogically one might expect
abhinc unum annum, but that expression doesn't appear in the literature.
Anno proximo/priore isn't exactly equivalent.
Anyway, I believe inclusive reckoning only applies to ordinal numerals, and even then not always. For example,
nudius tertius literally means something like "now it being the third day [from then]", where "then" counts as the first day. The same applies to the official reckoning of dates with
ante diem. On the other hand,
tertio quoque anno can apparently either be equivalent to
altero quoque anno "every other year" or really mean "every third year", so there's obviously some confusion on this point even among the Romans.
At the beginning of his 2nd Philippic Cicero says that there was no enemy of the republic within the last 20 years (
his annis viginti) that had not declared war against him, no doubt referring to the Catilinarian conspiracy in 63 BC as the limit. It was supposed to be delivered in October of 44 BC, which means he's counting the present year inclusively, but I suspect it's a vague expression that applies the years as counted according to the Roman new year rather than precisely indicating how many years from the day it was given.
At the end he says
Etenim, si abhinc annos prope viginti hoc ipso in templo negavi posse mortem immaturam esse consulari, quanto verius nunc negabo seni! He's referring here to the delivery of one of the Catilinarian orations given in 63, but he more precisely designates the interval as "almost 20 years ago". I don't know which oration he means, so there could be a mere difference in months that accounts for the
prope, but I suspect he isn't counting inclusively here.
(BTW Desessard accustomed me to regard annō proximō as next year.)
Depending on context it could refer to either the last year or the next year.