Key takeaways
- BCE means Before Common Era and CE means Common Era; they map exactly onto BC and AD, using the same year numbers.
- The change is one of terminology rather than of the underlying scale — 1500 CE and AD 1500 are the same year.
- There is no year zero in this system: 1 BCE is followed directly by 1 CE, which adds a year to any span crossing the boundary.
- Centuries are counted from 1, which is why the twentieth century runs from 1901 to 2000 rather than 1900 to 1999.
- Astronomers use a different convention that does include a year zero, which is why astronomical and historical dates can differ by one.
BCE means Before Common Era and CE means Common Era. They use exactly the same year numbers as BC and AD. 1500 CE and AD 1500 are the same year. Nothing about the scale changed; only the wording did.
That is the whole answer to the most-asked question, and the more interesting material is in what the system does strangely.
Why the terminology changed
The older abbreviations are explicitly religious in their wording. A dating system used across cultures, religions and academic disciplines benefits from terminology that does not embed one tradition's framing.
Because the numbering was already shared, the change cost nothing: no dates had to be recalculated, and any text using one system can be read by someone familiar with the other.
Both remain in use. Academic history, archaeology and museums have largely adopted BCE and CE; general writing still uses both.
The missing year zero
Here is the genuinely odd part. The sequence runs 1 BCE, then 1 CE. There is no year zero.
This is an artefact of when the scheme was devised — before zero was in common use as a number in European arithmetic. Nobody chose to omit it; the concept was simply not available in that form.
The consequence is arithmetic:
To calculate a span crossing the boundary, add the two year numbers and subtract one.
From 50 BCE to 50 CE is 99 years, not 100. From 1 BCE to 1 CE is one year.
Get this wrong and every duration you calculate across the boundary is off by one — a small error that compounds embarrassingly in a timeline.
Why centuries start in years ending 01
The same counting convention produces the other classic confusion.
Counting starts at year 1, not year 0. So:
| Century | Runs from | To |
|---|---|---|
| 1st | 1 | 100 |
| 2nd | 101 | 200 |
| 20th | 1901 | 2000 |
| 21st | 2001 | 2100 |
Strictly, the twentieth century ran from 1901 to 2000. In ordinary usage "the 1900s" means 1900–1999, and most people mean that when they say "the twentieth century".
Both usages are common. The strict version is the one that follows from the counting rule; the loose version is the one people actually mean. It is worth knowing which you are reading.
The astronomical exception
Astronomers use a different convention that does include a year zero: 1 BCE is written as year 0, 2 BCE as −1, and so on.
The reason is practical. Astronomical calculations involve arithmetic across the boundary constantly, and a system with a gap in it makes every formula more complicated.
The consequence is that an astronomical year number and a historical year number for the same BCE date differ by one. The U.S. Naval Observatory's introduction to calendars sets out the conventions and why they differ.
If you are comparing a date from an astronomical source with one from a historical source, this is the first thing to check.
Why the notation matters for reading history
Two practical reasons beyond avoiding arithmetic errors.
BCE dates count backwards. 500 BCE is earlier than 400 BCE. This is obvious when stated and surprisingly easy to invert when reading quickly, particularly in a list.
Precision falls off with distance. A date given as "c. 3000 BCE" carries far wider uncertainty than one given as "44 BCE". The notation does not encode this, so a timeline can present a well-attested date and an archaeological estimate with identical confidence. How historians date events covers where those dates come from and how confident anyone can be.
Where this fits in the wider system
BCE/CE is a numbering convention layered on top of a calendar. The calendar itself — which days fall where, how leap years work, and the fact that the calendar in use changed at different times in different places — is a separate question, covered in how calendars and dating systems work.
For the broader shape of what those dates are organising, the major eras of world history covers how periods are defined and why the boundaries are arguments rather than facts.
Where our app fits
World History Timeline Sim presents historical events on a timeline, which means the conventions above are load-bearing: an interface that puts BCE and CE dates on one scale has to handle the missing year zero correctly or its intervals are wrong.
It is free to download on Google Play, supported by ads, with optional in-app purchases and an Everyone content rating.
If you want to evaluate historical claims rather than browse them, primary versus secondary sources covers the distinction that matters most, and the rest of what we make in this area is under education and brain games.
Good to know
Frequently asked questions
What do BCE and CE stand for?
Why did academic writing move to BCE and CE?
Why is there no year zero?
How do you calculate a span across the boundary?
Why does the twentieth century start in 1901?
Sources
- Introduction to Calendars (opens in a new tab)U.S. Naval Observatory, Astronomical Applications Department — accessed
- Time and Frequency Division (opens in a new tab)National Institute of Standards and Technology (NIST) — accessed