How to translate BC dates into Holocene Era

As you may know, I translate Wikipedia articles into Holocene Era. While doing it for some time now I came up with a bunch of formulas as well as some tips and tricks that may not be obvious to you at first. But if you are going to translate dates from BC to HE, you should know about them.

Precise translation of years

The most common mistake I see people making is thinking that to convert a date from BC to HE you just subtract it from 10 000. To understand why it’s wrong let’s visualize BC dates of previous decamillennium on top of corresponding HE dates:

BC and HE 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. So the correct formula for translating a date from BC to HE is: 10 001 – x

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

Imprecise translation of years

Sometimes you can use formula 10 000 – x though. I call it imprecise translation. For example let’s say I’m translating a Wikipedia article and there is a photo of some ancient statue and the caption says that this statue is dated roughly to years 70-40 BC. If I translate these dates using the first formula (precise translation) I’d get 9931-9961. In this case it would be better to use imprecise translation that would give 9930-9960 instead. On one hand, one year difference is insignificant since the statue can’t be dated precisely anyway, and on another hand I get nice round numbers instead of weird numbers ending with 1s (which give an illusion that these dates are precise).

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

Second, there are cases when a date is both round and imprecise, but you still should translate it precisely. For example it is said that Caesar was born circa 100 BC. 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 BC, another suggesting he was born in 100 BC. 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 BC 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 BC that is round and probably not precisely known, and I translate this date imprecisely, effectively shifting it by one year, that may change his age that is mentioned in the article as well. Am I allowed to change person’s age? 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.

As a reader of my wiki project (or any text translated into HE) you should keep in mind that if a HE year ends with 1 or 6, it may be a precise translation of a round (and probably not known precisely) year BC ending with 0 or 5.

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

With decades it’s a bit more complicated but let’s start with simple stuff first. Let’s look at the 1st century BC or 100th century HE:

The pairs of numbers always give a sum of 9990. So the formula for translating decades is 9990 – x. And it works for any century. For example 320s BC would be translated into 9670s (9990 – 320).

And this is a picture of any century BC:

When you need to translate phrases like ‘20s of 4th century BC’ the short formula that will work for any century is: 90 – x

But you should be aware that decades BC and decades HE don’t match perfectly. Actually they only have an 80% match.

It happens because our decades are shifted by one year. For example a proper third decade of a given century is comprised of years 21 to 30. And a proper eight decade is comprised of years 71 to 80. Proper decade is a term I just invented. If we used decades this way there would be a perfect match between BC and HE decades just like with millennia and centuries.

But instead we call those decades 20s and 70s. 20s are comprised of years 20 to 29, and 70s are comprised of years 70 to 79. So, each decade is shifted by one year relative to corresponding proper decade. And since BC and HE decades are shifted in different directions we have a mismatch of 2 years.

So, you should be careful when translating decades. Especially you should be careful when figuring out whether or not a particular date belongs to a particular decade.

Categorizing dates by decades

This is the most weird stuff. The best way to explain it is to give an example. Let’s say I’m translating a Wikipedia article about some person who was born in year 320 BC. On the bottom of the same article there is a list of categories, and one of those categories is called ‘People born in 320s BC’. I need to translate his birth date as well as the category name.

When I translate his year of birth, it becomes 9681. When I translate the decade 320s BC using the formula 9990 – x, I get 9670s (but the person was born in 9681). The reason this happens is because of the mismatch between BC and HE decades I already told you about. Even if he was born in 321 BC (9680) it would still be in 9680s. See the picture above to visualise it.

So, when you want to categorize a BC date that ends with 0 or 1 by decade you should use formula: 10000 – x, where x is your decade (not date itself). In our example that would be: 10000 – 320 = 9680.

The short formula that would work for every century is 100 – x. This formula would give a weird result for 0s decade, but thankfully I’ve never encountered phrases like ‘in 0s of 4th century BC’.

Automating date conversions

See how easy it is to translate dates with my date conversion tool.


In conclusion let me just list all the formulas.

Precise translation of years: 10 001 – x

Imprecise translation of years: 10 000 – x

Millennia: 11 – x

Centuries: 101 – x

Decades: 9990 -x

Decades, short: 90 – x

Categorization of a date ending with 0 or 1 by decade: 10 000 – x (where x is a decade, not the date)

The same, but short version good for every century: 100 – x

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