In this article I’ll show you most of the formulas I use in my Old Era Chrome extension for date conversions between Christian timeline and the Old Era timeline.

If you haven’t installed the extension yet, you can do it here.

## Precise translation of years

To translate any year BC into an OE year I subtract it from 10 001. The formula is: **10 001 – x**

To see why this is, let’s visualise OE dates next to the corresponding BC dates:

BC and OE dates form pairs (1 and 10000, 2 and 9999, 3 and 9998, etc), and the sum of each pair is constant and equal to 10 001. That’s where the number 10001 comes from in this formula: **10 001 – x**

For example, Caesar was murdered in 44 BC. In OE that would be (10 001 – 44) 9957.

## Imprecise translation of years

Sometimes I can use formula **10 000 – x** though. I call it imprecise translation. Let’s take for example the first historical event, the unification of Egypt, that happened, according to some historians, in 3100 BCE.

If we translate this year with the precise formula, we will get (10 001 – 3100) 6901. The ‘1’ at the end creates an impression that we are talking about a precisely dated event even though, of course, the original year 3100 BCE was a rough estimate. It would be better if we could convert 3100 BCE to year 6900 instead.

In such cases I use the formula: **10 000 – x**

It is applied to round years like 3100 BCE. Sometimes to years like 2175 BCE. Basically if a year ends with 0 or 5 I may use imprecise translation.

But I have to be careful with imprecise translation. First, if a BCE year is round it doesn’t necessarily mean that it is not precise. For example Egypt was annexed by Rome in 30 BCE. That is a precisely known date even though the year is round, so it should be translated precisely and become the year 9971.

Second, there are cases when a date is both round and imprecise, but I still translate it precisely. For example it is said that Caesar was born circa 100 BCE. But it doesn’t mean that since historians didn’t know the precise date, they just picked a nice and round one. Actually, there are a couple of hypotheses of when he might have been born, one suggesting Caesar was born in 102 BCE, another suggesting he was born in 100 BCE. That means that he was born either in 9899 or 9901. With imprecise translation we’d get year 9900 for his birth, which is the same as year 101 BCE and does not agree with any of the two hypotheses.

Third, a lot of the times I translate years precisely just to not cause a mess. For example if some person was born in some year BCE that is round and probably not precisely known, and I translate that date imprecisely, effectively shifting it by one year, that may change his age that is usually mentioned in the article as well. Also, if I shift his year of birth I’d better be consistent across multiple articles that may mention this person.

For these reasons I tend to translate birth-dates precisely even if they are not precisely known.

A lot of times I use precise or imprecise translation based on my intuition. Also, the less I know about some particular part of history, the more I tend to use precise translation.

What to do, if you think I made a mistake by translating a particular date imprecisely while I should have translated it precisely or vice versa? There are two things you can do.

You may tell me about it, and if you convince me that I made a mistake, I’ll fix it.

If you don’t want to rely on my judgment and you are OK with having all the years translated precisely, just select ‘always translate years precisely’ in the pop-up menu of the extension. In this case you’ll have a lot of years ending with 1s, like year 6901.

## Translation of millennia and centuries

Everything is much simpler with millennia and centuries. There is only one way to translate them.

For millennia the formula is: **11 – x**

For example, 2nd millennium BC is (11 – 2) 9th millennium.

For centuries the formula is **101 – x**

For example, 4th century BC is (101 – 4) 97th century.

## Translation of decades

When it comes to decades, you should know that unlike years, centuries and millennia, decades BCE and decades OE don’t match perfectly. They are kind of shifted relative to each other. Let’s take for example 520s BCE:

You may say that 520s BCE correspond roughly to 9470s OE. You can use the formula, **9990 – x** to translate each decade this way. But there is a problem with this approach.

A lot of times you’ll see phrases like ‘early 520s BCE’ or ‘late 520s BCE’. If I just translate this decade to 9470s, I’ll need to somehow convey to the readers that although they see the phrase ‘early 9470s’, they should mentally exclude years 9470 and 9471 from that decade. And when they see the phrase ‘late 9470s’, they need to include years 9480 and 9481.

Also sometimes an article on Wikipedia may mention some event X that happened in 520s BCE. If I don’t know much about that part of history I would start wondering what if that event actually happened in years 521 and/or 520 BCE -or 9480 and 9481 after translation – and in this case the event would not belong to 9470s.

In every such case I would have to conduct a small investigation about the event X. So I came up with a solution that solves all these problems:

I call it Christian decades. Instead of trying to find the closest OE decade that will never match with the BCE decade perfectly, I use the BCE decade itself, translating it precisely, as a range of years.

It may look crazy for the first few seconds you look at it, until you realise that the same pattern repeats for every decade. The first year of any decade ends with ‘2’, and the last one ends with ‘1’. For example, 9902/11, 9912/21 and so on. And so you can get used to it pretty fast. If the decade touches two centuries I write it down without shortening the last year: ‘9492/9501 decade’.

The only two weird decades are 0s BCE or 9992/10000 that has only 9 years in it, and the very first year of the Old Era that is a ‘decade’ containing only one year. The latter is never used in actual articles on Wikipedia, though.

It is probably useful to note that this ‘weird’ notation I use for decades is not and intrinsic feature of the Old Era. Let’s say you are a historian and you are writing a history textbook that uses OE dates. If you create your text from the top of your head (not translating someone else’s text) you can use decades like 9470s and they would mean exactly what one would expect when looking at them (in our example 9470s would mean years from 9470 to 9479).

It is only when you translate a text that originally used BCE dates that you have to translate decades as ranges of years. This approach ensures that you convey exactly what the original author of the text meant by using a specific BCE decade.