1. Our modern system of identifying the years was first introduced in AD 525 by Dionysius Exiguus (= “Denis the Little”), who calculated historical dates by reference to the year of Christ’s birth. Since the Roman numerical system did not include the concept of “zero,” this year was designated year 1. All dates prior to Christ’s birth were identified as BC (= “before Christ”) and all dates after Christ’s birth were identified as AD ( = “Anno Domini”—”in the year of the Lord”). This system was made official by Pope Gregory XIII in 1582.

2. Unfortunately, Dionysius’s calculations were a few years off. Modern scholars date Christ’s birth to a few years before the start of the Christian era (generally sometime between 6–4 BC) based on historical and astronomical data.

3. The abbreviation BC always appears after the date; the abbreviation AD always appears before the date.

4. A time line based on this system looks like this:

2000 BC – 1000 BC – 500 BC – ✙ – AD 500 – AD 1000 – AD 2000

5. BC dates go in descending order (thus 2000 BC is much more ancient than 500 BC); AD dates go in ascending order (thus AD 2000 is much more recent than AD 500).

6. A century consists of precisely 100 years—not 99, not 101. Therefore, when counting centuries, the years 100–1 BC refer to the first century BC: the year designated “100” is the first year of the new century. The years AD 1-100 refer to the first century AD: the year designated “100” is the last year of the old century. Therefore, if one states that an event occurred in the eighth century BC, he or she is referring to the years 800–701 BC. If one states that an event occurred in the eighth century AD he or she is referring to the years AD 701–800.

7. Because many dates from the ancient world are approximate, the abbreviation ca. or c. (meaning circa = “around”) will appear before many dates. Example: ca. 300 BC.

8. Many biblical scholars use the abbreviations BCE (“before the Common Era”) and CE (“Common Era”) in deference to those who do not come from a Christian tradition.

(This material was adapted from Dr. Susan Pigott of Hardin-Simmons University)