Archaeologists, geologists, and other palaeoscientists use different systems for numbering years in the distant past. For example, the year 10,000 BCE is 11,950 Before Present or 11.95 ka. It is usually fine to store years as a plain numeric vector in R, but sometimes it helps to be explicit about which system is being used:
The era package helps in these cases by providing classes which define the ‘era’ associated with a vector of years and functions for formatting, combining, and transforming years with different eras. This vignette is an introduction to the main features of the package.
Vectors of years with an era are represented by the yr (era_yr) class, which is constructed with yr():
yr(c(10000, 11000, 12000), "BP")
#> # BP years <yr[3]>:
#> [1] 10000 11000 12000
#> # Era: Before Present (BP): Gregorian years (365.2425 days), counted backwards from 1950The first argument is a numeric vector of years. These can be integers or doubles.
The second argument, era, defines the numbering system associated with the years. This is an object of class era which defines the parameters of the calendar, epoch and time scale. Most of the time, you can simply specify the abbreviated label of the era, which will be looked up in the standard eras defined by eras():
yr(c(10000, 11000, 12000), "BCE")
#> # BCE years <yr[3]>:
#> [1] 10000 11000 12000
#> # Era: Before Common Era (BCE): Gregorian years (365.2425 days), counted backwards from 1
yr(c(10000, 11000, 12000), "uncal BP")
#> # uncal BP years <yr[3]>:
#> [1] 10000 11000 12000
#> # Era: uncalibrated Before Present (uncal BP): radiocarbon years (NA days), counted backwards from 1950
yr(c(10000, 11000, 12000), "ka")
#> # ka years <yr[3]>:
#> [1] 10000 11000 12000
#> # Era: kiloannum (ka): 1000 Gregorian years (365.2425 days), counted backwards from 1950yr_era() returns details of the era associated with a yr vector:
neolithic <- yr(11700:7500, "BP")
yr_era(neolithic)
#> <era[1]>
#> [1] Before Present (BP): Gregorian years (365.2425 days), counted backwards from 1950yr_era(), and its pipe-friendly alias yr_set_era(), can also be used to set the era of an existing object:
chalcolithic <- 7500:6000
yr_era(chalcolithic) <- yr_era(neolithic)
yr_era(chalcolithic)
#> <era[1]>
#> [1] Before Present (BP): Gregorian years (365.2425 days), counted backwards from 1950Note that this only updates the vector’s era attribute; it doesn’t change the data itself. To convert years from one era to another, you need to use the yr_transform() function.
yr vectors fit nicely into tables, both base data frames and tibbles:
postglacial <- tribble(
~period, ~start_ka,
"Late Holocene", 4.2,
"Mid Holocene", 8.326,
"Early Holocene", 11.7,
"Younger Dryas", 12.9,
"Bølling-Allerød", 14.7,
"Heinrich 1", 17.0
)
postglacial |>
mutate(start_ka = yr(start_ka, "ka"))
#> # A tibble: 6 × 2
#> period start_ka
#> <chr> <yr>
#> 1 Late Holocene 4.2 ka
#> 2 Mid Holocene 8.326 ka
#> 3 Early Holocene 11.7 ka
#> 4 Younger Dryas 12.9 ka
#> 5 Bølling-Allerød 14.7 ka
#> 6 Heinrich 1 17 kaera includes built-in definitions of many time scales and year numbering systems from contemporary and historic calendars. eras() returns the full list of built-in definitions. You can use any definition in this list by passing its abbreviated label to era() or as the era argument of yr() or any other function in the package:
era("BP")
#> <era[1]>
#> [1] Before Present (BP): Gregorian years (365.2425 days), counted backwards from 1950
yr(10000, "BP")
#> # BP years <yr[1]>:
#> [1] 10000
#> # Era: Before Present (BP): Gregorian years (365.2425 days), counted backwards from 1950
yr_transform(yr(10000, "BP"), "BCE")
#> # BCE years <yr[1]>:
#> [1] 8051
#> # Era: Before Common Era (BCE): Gregorian years (365.2425 days), counted backwards from 1If you need to use a time scale that is not in this list, you can define it yourself with era(). Suggestions for new eras to include in the package are also welcome; please create an issue on GitHub with suggestions.
| label | epoch | name | unit | scale | direction | this_year |
|---|---|---|---|---|---|---|
| BP | 1950.0000 | Before Present | Gregorian years (365.2425 days) | 1e+00 | -1 | -72 |
| cal BP | 1950.0000 | Before Present | Gregorian years (365.2425 days) | 1e+00 | -1 | -72 |
| BC | 1.0000 | Before Christ | Gregorian years (365.2425 days) | 1e+00 | -1 | -2021 |
| BCE | 1.0000 | Before Common Era | Gregorian years (365.2425 days) | 1e+00 | -1 | -2021 |
| AD | 0.0000 | Anno Domini | Gregorian years (365.2425 days) | 1e+00 | 1 | 2022 |
| CE | 0.0000 | Common Era | Gregorian years (365.2425 days) | 1e+00 | 1 | 2022 |
| a | 1950.0000 | annum | Gregorian years (365.2425 days) | 1e+00 | -1 | -72 |
| ka | 1950.0000 | kiloannum | Gregorian years (365.2425 days) | 1e+03 | -1 | 0 |
| Ma | 1950.0000 | megaannum | Gregorian years (365.2425 days) | 1e+06 | -1 | 0 |
| Ga | 1950.0000 | gigaannum | Gregorian years (365.2425 days) | 1e+09 | -1 | 0 |
| kya | 1950.0000 | thousand years ago | Gregorian years (365.2425 days) | 1e+03 | -1 | 0 |
| mya | 1950.0000 | million years ago | Gregorian years (365.2425 days) | 1e+06 | -1 | 0 |
| bya | 1950.0000 | billion years ago | Gregorian years (365.2425 days) | 1e+09 | -1 | 0 |
| b2k | 2000.0000 | years before 2000 | Gregorian years (365.2425 days) | 1e+00 | -1 | -22 |
| uncal BP | 1950.0000 | uncalibrated Before Present | radiocarbon years (NA days) | 1e+00 | -1 | NA |
| RCYBP | 1950.0000 | Radiocarbon Years Before Present | radiocarbon years (NA days) | 1e+00 | -1 | NA |
| bp | 1950.0000 | Before Present (uncalibrated) | radiocarbon years (NA days) | 1e+00 | -1 | NA |
| bc | 1950.0000 | Before Christ (uncalibrated) | radiocarbon years (NA days) | 1e+00 | -1 | NA |
| bce | 1950.0000 | Before Common Era (uncalibrated) | radiocarbon years (NA days) | 1e+00 | -1 | NA |
| ad | 1950.0000 | Anno Domini (uncalibrated) | radiocarbon years (NA days) | 1e+00 | 1 | NA |
| ce | 1950.0000 | Common Era (uncalibrated) | radiocarbon years (NA days) | 1e+00 | 1 | NA |
| AD O.S. | 0.0000 | Anno Domini (Old Style) | Julian years (365.25 days) | 1e+00 | 1 | 2022 |
| BC O.S. | 1.0000 | Before Christ (New Style) | Julian years (365.25 days) | 1e+00 | 1 | 2021 |
| H | 621.5366 | Hijra | Islamic lunar years (354.36708 days) | 1e+00 | 1 | 1444 |
| AH | 621.5366 | Anno Hegirae | Islamic lunar years (354.36708 days) | 1e+00 | 1 | 1444 |
| BH | 622.5366 | Before the Hijra | Islamic lunar years (354.36708 days) | 1e+00 | -1 | -1443 |
| SH | 621.2190 | Solar Hijri | Nowruz years (365.2424 days) | 1e+00 | 1 | 1401 |
| BSH | 622.2190 | Before Solar Hijri | Nowruz years (365.2424 days) | 1e+00 | -1 | -1400 |
| AM | -3760.2361 | Anno Mundi | Hebrew lunisolar years (365.246822205978 days) | 1e+00 | 1 | 5783 |
| HE | -10000.0000 | Holocene Era | Gregorian years (365.2425 days) | 1e+00 | 1 | 12022 |
| BHE | -10000.0000 | Before Holocene Era | Gregorian years (365.2425 days) | 1e+00 | -1 | -12022 |
| AL | -4000.0000 | Anno Lucis | Gregorian years (365.2425 days) | 1e+00 | 1 | 6022 |
| ADA | -8000.0000 | After the Development of Agriculture | Gregorian years (365.2425 days) | 1e+00 | 1 | 10022 |
Eras are defined by the era class with the following parameters:
1) or backwards (-1) from the epochThese parameters are passed to era() to construct an era object. epoch, unit, scale, and direction determine the transformation between eras; label and name are purely descriptive.
You can define arbitrary eras by using the era() function directly:
era("T.A.", epoch = -9021, name = "Third Age", direction = 1)
#> <era[1]>
#> [1] Third Age (T.A.): Gregorian years (365.2425 days), counted forwards from -9021As long as all the parameters are specified correctly, user-defined eras can also be used in yr_transform().
yr_transform()
Use yr_transform() to convert between eras:
postglacial |>
mutate(start_ka = yr(start_ka, "ka")) |>
mutate(start_bp = yr_transform(start_ka, era("BP")),
start_bce = yr_transform(start_ka, era("BCE")))
#> # A tibble: 6 × 4
#> period start_ka start_bp start_bce
#> <chr> <yr> <yr> <yr>
#> 1 Late Holocene 4.2 ka 4200 BP 2251 BCE
#> 2 Mid Holocene 8.326 ka 8326 BP 6377 BCE
#> 3 Early Holocene 11.7 ka 11700 BP 9751 BCE
#> 4 Younger Dryas 12.9 ka 12900 BP 10951 BCE
#> 5 Bølling-Allerød 14.7 ka 14700 BP 12751 BCE
#> 6 Heinrich 1 17 ka 17000 BP 15051 BCEThis function implements a generic algorithm for transforming years based on the era parameters described above. This means that, with a few exceptions (see invalid transformations, you can transform between any two eras that can be described by the era class.
By default, era transformations are exact:
yr(500000, "BCE") |>
yr_transform(era("ka"))
#> # ka years <yr[1]>:
#> [1] 501.949
#> # Era: kiloannum (ka): 1000 Gregorian years (365.2425 days), counted backwards from 1950Often, this precision is not necessary. For example, when converting years between calendar- and present-based eras, the 72 year difference between the formal definition of “Present” and the actual present is rarely significant on a geologic time scale. Use the precision argument of yr_transform to get rounded results:
yr(10000, "BP") |>
yr_transform(era("BCE"), precision = 1000)
#> # BCE years <yr[1]>:
#> [1] 8000
#> # Era: Before Common Era (BCE): Gregorian years (365.2425 days), counted backwards from 1
yr(500000, "BCE") |>
yr_transform(era("mya"), precision = 0.1)
#> # mya years <yr[1]>:
#> [1] 0.5
#> # Era: million years ago (mya): 1000000 Gregorian years (365.2425 days), counted backwards from 1950Some transformations are not possible. Notably, the length of a ‘radiocarbon year’ is not well defined on a calendar time scale without calibration. Eras that use non-calendar year unit are represented with an NA and will cause an error if passed to yr_transform():
era_unit(era("uncal BP"))
#> <era_year[1]>
#> [1] radiocarbon years (NA days)
yr_transform(yr(9000, "uncal BP"), era("cal BP"))
#> Error in `yr_transform()`:
#> ! Cannot transform uncalibrated Before Present to Before Present years:
#> ✖ Calendar length of a radiocarbon year is undefined.c14_calibrate() from the stratigraphr package implements radiocarbon calibration with yr objects.
Conversion between eras that both have an NA unit are also an error, following the R convention that NA == NA is NA. In other words, we don’t know whether two non-calendar units are the same non-calendar unit. This means that it is not possible to use yr_transform() to convert bp (radiocarbon years Before Present) to bce (radiocarbon years before the Common Era) years, for example.
The yr class is based on vctrs, ensuring type- and size-stable computations. For example, you can do arithmetic with year vectors:
a <- yr(1500, "CE")
b <- yr(2020, "CE")
b - a
#> # CE years <yr[1]>:
#> [1] 520
#> # Era: Common Era (CE): Gregorian years (365.2425 days), counted forwards from 0But only when they have the same era:
c <- yr(0.5, "ka")
b - c
#> Error in `vec_cast.era_yr.era_yr()`:
#> ! Can't convert `y` <yr (ka)> to <yr (CE)>.
#> Reconcile eras with yr_transform() first.Note that, when comparing eras, only the parameters significant to the transformation are considered (i.e. not label or name). This means that it is possible to combine year vectors with differently-named but functionally equivalent eras, for example era("BP") and era("cal BP"), although doing so will print a warning about the loss of information:
era("BP") == era("BC")
#> [1] FALSE
era("BP") == era("cal BP")
#> [1] TRUE
yr(1000, "BP") + yr(1000, "cal BP")
#> Warning: eras have different label or name parameters.
#> # BP years <yr[1]>:
#> [1] 2000
#> # Era: Before Present (BP): Gregorian years (365.2425 days), counted backwards from 1950Years will be coerced to a plain numeric vector if a computation means their era no longer makes sense:
a * b
#> [1] 3030000