Why 1 BC Is Followed by AD 1
Introduction
In the previous article, we examined the meaning of BC and AD and how the Christian calendar was developed. One of the most unusual features of this calendar system is that it does not include a year zero.
This detail often creates confusion when calculating dates across the boundary between BC and AD. Understanding why this happens is essential for studying the historical timeline of Jesus and other ancient events.
The Transition from BC to AD
The BC/AD calendar moves directly from 1 BC to AD 1. The sequence looks like this:
- 3 BC
- 2 BC
- 1 BC
- AD 1
- AD 2
- AD 3
There is no year 0 between 1 BC and AD 1.
This means the calendar moves directly from the final year before Christ to the first year counted in the era associated with Christ.
Why Ancient Calendars Had No Year Zero
The absence of a year zero comes from the way numbers were used in the ancient world. The concept of zero as a number was not commonly used in European mathematics during late antiquity and the early medieval period.
When the monk Dionysius Exiguus created the AD system in the sixth century, European scholars used Roman numerals, which had no symbol for zero.
Because of this, the calendar naturally moved from 1 BC directly to AD 1 (Richards, 2013).
Mathematical Implications
The missing year zero means that calculating time periods across the BC/AD boundary requires careful attention.
For example:
- From 1 BC to AD 1 is one year, not two.
- From 2 BC to AD 2 is three years, not four.
Historians and astronomers must therefore adjust their calculations when working with ancient chronologies.
For scientific purposes, astronomers sometimes use a system that includes year 0 to simplify calculations. In this astronomical system:
- 1 BC becomes year 0
- 2 BC becomes year –1
However, this system is not used in ordinary historical writing (Blackburn and Holford-Strevens, 1999).
Why This Matters for the Life of Jesus
The absence of a year zero plays an important role when discussing the timeline of Jesus’ life.
If Jesus was born several years before AD 1, historians must carefully count years across the BC/AD boundary to estimate His age during key events such as:
- the beginning of His ministry
- the crucifixion
Because the calendar lacks year zero, age calculations across this boundary can easily produce incorrect results if the rule is not understood.
A Simple Way to Visualise the Timeline
The calendar sequence can be visualised like this:
6 BC → 5 BC → 4 BC → 3 BC → 2 BC → 1 BC → AD 1 → AD 2 → AD 3
This continuous sequence shows that BC and AD are directly connected, with no gap between them.
Conclusion
The absence of a year zero is a small detail with important consequences. Because ancient numbering systems did not include zero, the calendar moves directly from 1 BC to AD 1.
This feature explains why calculations involving the birth and death of Jesus require careful attention to chronology.
In the next article, we will explore an important historical question: Was Jesus actually born in AD 1, or did His birth occur several years earlier?
References
Blackburn, B. and Holford-Strevens, L., 1999. The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-Reckoning. Oxford: Oxford University Press.
Richards, E.G., 2013. Mapping Time: The Calendar and Its History. Oxford: Oxford University Press.