system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 — Why is the Christian year (2012) used by 
everyone around the world, including Jews and 
Hindus and Muslims?
gpt
In short, because of Western-European cultural imperialism and dominance.

In what we now call the year 525, a Christian monk named Dennis the Short (Dionysius Exiguus) devised an era system based from the birth of Jesus, and deemed (erroneously) that Jesus was born five hundred and twenty-five years ago from that standpoint. He prefaced all years from the year "1" onward with the phrase *Anno Domini Nostri Iesu Christi* (Latin: "In the year of our Lord, Jesus Christ"), which eventually became shortened to *Anno Domini*, and then abbreviated as we know it today; *AD*. That is why we call the current year 2012 AD.

In the 8th century, Saint Bede the Venerable was calculating the dates for Easter, and came upon the necessity to tabulate years prior to 1 AD. He called those years *Ante Christum Natum* (Latin: "Before the birth of Christ"). He knew not of the mathematical concept of "zero", so the year immediately prior to 1 AD was deemed 1 ACN (later translated to English as "Before Christ"; abbreviated to 1 BC).

This system quickly became the predominant year era system in the Western world, and as the West's influence spread, so did the common use of the era. There are other eras in existence (Jewish year—*5772 AM*; Islamic year—*1433 AH*), but they are secondary to the Christian era for convenient civil use in virtually every civilized country on Earth.