# Years with an era

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:

- When you have data that mixes different systems
- When you want to transform years between different systems
- When you need to do arithmetic with years

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.

    library("era")
    library("tibble")
    library("dplyr")

## Years with an era

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 1950

The 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 1950

`yr_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 1950

`yr_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 1950

Note 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 ka

## Era definitions

era 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 1

If 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.

### List of built-in eras

label | epoch | name | unit | scale | direction | this_year
--- | --- | --- | --- | --- | --- | ---
BP | 1950.0000 | Before Present | Gregorian years (365.2425 days) | 1e+00 | -1 | -76
cal BP | 1950.0000 | Before Present | Gregorian years (365.2425 days) | 1e+00 | -1 | -76
BC | 1.0000 | Before Christ | Gregorian years (365.2425 days) | 1e+00 | -1 | -2025
BCE | 1.0000 | Before Common Era | Gregorian years (365.2425 days) | 1e+00 | -1 | -2025
AD | 0.0000 | Anno Domini | Gregorian years (365.2425 days) | 1e+00 | 1 | 2026
CE | 0.0000 | Common Era | Gregorian years (365.2425 days) | 1e+00 | 1 | 2026
a | 1950.0000 | annum | Gregorian years (365.2425 days) | 1e+00 | -1 | -76
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 | -26
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 | 2026
BC O.S. | 1.0000 | Before Christ (New Style) | Julian years (365.25 days) | 1e+00 | 1 | 2025
H | 621.5366 | Hijra | Islamic lunar years (354.36708 days) | 1e+00 | 1 | 1448
AH | 621.5366 | Anno Hegirae | Islamic lunar years (354.36708 days) | 1e+00 | 1 | 1448
BH | 622.5366 | Before the Hijra | Islamic lunar years (354.36708 days) | 1e+00 | -1 | -1447
SH | 621.2190 | Solar Hijri | Nowruz years (365.2424 days) | 1e+00 | 1 | 1405
BSH | 622.2190 | Before Solar Hijri | Nowruz years (365.2424 days) | 1e+00 | -1 | -1404
AM | -3760.2361 | Anno Mundi | Hebrew lunisolar years (365.246822205978 days) | 1e+00 | 1 | 5786
HE | -10000.0000 | Holocene Era | Gregorian years (365.2425 days) | 1e+00 | 1 | 12026
BHE | -10000.0000 | Before Holocene Era | Gregorian years (365.2425 days) | 1e+00 | -1 | -12026
AL | -4000.0000 | Anno Lucis | Gregorian years (365.2425 days) | 1e+00 | 1 | 6026
ADA | -8000.0000 | After the Development of Agriculture | Gregorian years (365.2425 days) | 1e+00 | 1 | 10026

### Defining other eras

Eras are defined by the `era` class with the following parameters:

- label: an abbreviated label that uniquely identifies the era
- name: the full name of the era
- epoch: the origin year from which years are counted (in Gregorian astronomical years)
- unit: the unit of years counted, defined as its length in solar days
- scale: the number of years represented by one unit
- direction: whether years are counted forwards (`1`) or backwards (`-1`) from the epoch

These 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 -9021

As long as all the parameters are specified correctly, user-defined eras can also be used in `yr_transform()`.

## Converting between eras: `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 BCE

This 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.

### Transformation precision

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 1950

Often, this precision is not necessary. For example, when converting years between calendar- and present-based eras, the 76 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 1950

### Invalid transformations

Some 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.

## Arithmetic with year vectors

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 0

But 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 1950

Years will be coerced to a plain numeric vector if a computation means their era no longer makes sense:

    a * b
    #> [1] 3030000