As you already know, the abbreviations AD for “Anno Domini” (in the year of the Lord) and BC for “before Christ” indicate where a year occurs with respect to best known date for the birth of Jesus Christ. Here’s what else you should already know.
The Anno Domini system was invented in AD 525 by Dionysius Exiguus, a revered intellectual Romanian/Bulgarian monk living in Rome and serving on the Roman Curia. His model, also known as the Dionysian Era system, was widely adopted after Saint Bede the Venerable, an English monk, used it in AD 731 in his groundbreaking Ecclesiastical History of the English People (which also gained him the title “The Father of English History”). Dionysius assigned year AD 1 to his best guess for the birth year of Christ based on his own research. Interestingly, some historians have since disagreed with his estimation. By correlating historical events such as the rule of Herod and known comet appearances with Biblical text, they have concluded that it’s more likely Christ was born between 6 BC and 4 BC.
The historically accepted convention for writing the abbreviations is to place AD before the year number and BC after, for example, AD 1066 and 44 BC.
Interestingly, there is no year zero in the Anno Domini scheme, so the calendar goes from 1 BC straight to AD 1. This has significant implications for when a century or millennium begins. Since the first millennium began on AD 1, it follows that the second began on AD 1001 and the third on AD 2001, even if most people of age had our millennium parties on the evening of December 31, 1999. Just to be sure, some of us celebrated it twice.
If you’re put off by the religious overtones of the Dionysian Era system, you can use the more neutral and inclusive Common Era system. The term Common Era (CE) may be substituted for AD, and Before Common Era (BCE) for the years before Christ. CE and BCE are both written after the number, so AD 1944 is the same as 1944 CE, and 44 BCE is the same as 44 BC. Although this system has existed for hundreds of years, it never enjoyed the popularity of the Dionysian system. However, the twenty-first century has seen an uplift in Common Era usage in science, media and in school systems around the world, so be on the lookout.
But there is more you should know… In scientific texts, especially regarding carbon dating and the distant past, the abbreviation BP is used to indicate Before Present. An event that took place 25,000 years in the past is better described as having happened 25,000 years ago rather than 23,000 years before something else that happened just over 2000 years ago. Thus, 25,000 BP rather than 23,000 BC. However, since the “present” is always moving, BP is defined as before January 1, 1950 (AD/CE of course). So I guess in a few thousand years they’ll have to rethink this one too.