Ever since people started writing history, they’ve needed some way to locate events in time. Romans tied events to consular years. Two men were elected every year to serve in the office of consul. Roman historians said that a certain event took place in the year that Marcus Such and Such and Publius So and So were consuls.
Historians who lived in monarchies could date events to regnal years, that is, a time tied to the reign of a particular king. This battle was fought in the third year of King So and So.
Some Jewish and Christian writers referred to creation years. Various scholars believed they could date the creation of the earth from clues in the Bible. One commonly accepted date was October 7, 3761 BCE. Writers would use that date as a reference point.
Roman consular dating prevailed in the West until the year 541 CE when the Emperor Justinian I stopped appointing them.
The terms BC and AD were first used by a monk in about 525 CE. Its starting point was the year in which Jesus was thought to have been born. (Modern scholarship places the birth date of Jesus at 4 BCE.) Anything that happened before Year One is referred to as happening BC “Before Christ.” Anything after that date is said to have occurred AD “Anno Domini,” literally “in the year of the Lord,” i.e., after the birth of Jesus.
The traditional way of writing BC and AD dates is to put AD in front of the date and BC after it. For example, Julius Caesar was assassinated in 44 BC. Joan of Arc was executed in AD 1431.
Now that the various cultures of the world have become intertwined, a dating system based on a particular religious view is no longer appropriate. Most writers of history have adopted the notations BCE and CE.
BCE stands for “Before Common Era.” CE stands for “Common Era.” In this notation, Julius Caesar died in 44 BCE; Joan of Arc in CE 1431. The dividing date between BCE and CE is a Year One that coincides with the Consular Year One in which Gaius Caesar and Lucius Aemilius Paulus were consuls.
Different dating systems are still being used in various contexts. Muslims use a calendar in which Year One corresponds to CE 622, the year in which Mohammad took his followers from Mecca to Medina. Anything before that date is referred to as BH; anything after, as AH. The H stands for the Arabic word hijra, “migration.” AH stands for the Latin Anno Hegirae, “in the year of the Hijra.”
The Jewish calendar references its Year One as the year before the Creation, i.e. 3762 BCE. For Buddhists Year One corresponds to 543 BCE, the year Buddha died.