We think about history in terms of B.C.E. and A.D. now, but it hasn't always been this way. How did ancient peoples think about time in terms of cumulative years? When was year zero?
There is no year zero in the calendar you're referring to. BC = "before christ" and AD = "year of our lord", so 1BC switched right into 1AD. No zero.
Anyway, right now, this year, it is:
2016 — Christian calendar
1437 — Islamic calendar
5776 — Jewish calendar
1423 — Bengali/Assamese calendar
1377 — Burmese calendar
105 — Taiwanese calendar
Those are just the ones I've personally dealt with, but is by no means an exhaustive list.
I'm misquoting S J Gould here but he said calendars are entirely arbitrary rules, enforced consistently. And while we may think in terms of it being 2016 on Reddit where most of the userbase are Americans or Europeans, that's not the case in many places. In Taiwan its 105 first and 2016 after. In Assam you see the Bengali character in widespread use, and in Myanmar you'll see the Burmese calendar dates much more often than you'll see "2016" or the like.
Traditionally the Chinese calendar is based on a cycle of 60 years. The First Sino-Japanese War is known as the Jiawu (甲午) War, with 甲午 referring to the year it took place, 1894. 2014 was also Jiawu, being 120 years (2 cycles) later.
Then in addition to that you have Reign Years which are common in East Asia. The usual name is 年号, pronounced nengō in Japanese and niánhào in Mandarin. These could sometimes be a single name to refer to a ruler's entire reign, or, if the ruler was in the mood, multiple named periods throughout their rule. So 1674 in the Christian calendar was Kangxi 30 (康熙十三年) in terms of Chinese reign year, being the 30th (edit: 13th. swapped em) year of Kangxi's reign. Sometimes rulers picked reign names and sometimes they were known by those names, while in other cases the emperor's name is sufficient (so we can say "tenth year of so-and-so's rule").
These period names could also be reused, so taiyuan 太元 is both the name of a period during the Eastern Wu state's history (251-252CE) and the Eastern Jin dynasty (376-396CE).
Different calendars get based on different things. Taiwan's started in 1911, the date the Nationalists mark as their beginning. The Islamic calendar started with the move from Mecca to Medina by Muhammad around 622. The Burmese calendar put year zero at 638CE when it was created as a non-religious calendar. The Bengali/Assamese calendar is kinda a mess so I won't get into it, but you can read about it here.
The commonality is that in most cases, you have some important event, either the founding of a dynasty, the birth or other significant even in the life of a prophet, or just a government deciding that a change will help their claims of legitimacy. But in each case, it's a decision being made to mark a certain time as significant and being quite literally the start of a new era.
Arbitrary rules enforced consistently.
Hope that help. Let me know if you have follow up questions that aren't addressed by the FAQ posts you've been given.
Hi, you'll have a better chance of getting an in-depth answer if you can specify a region/culture and time period, but meanwhile you can get some ideas from here