Most of the world considers the current year to be 2018 (although I understand there are exceptions).
100 years ago it was 1918.
I doubt that 2017 years ago it was considered year 1.
When and how did the year 0 get decided upon, and at which point was the current year first accurate along this time keeping method?
It has already been answered on this very sub quite a number of times, here for instance.
https://www.reddit.com/r/AskHistorians/comments/p4gqx/when_did_we_start_using_the_bcad_dating_system/
If you don't want to check the link, the first use of the birth of Christ as year 1 (there was no year 0) was in 525 by Dionysius Exiguus. However this did not become widespread until the early IX century.