Years with an era

In era: Year-Based Time Scales

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

this_year <- as.integer(format(Sys.Date(), "%Y"))

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 {#yr}

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")

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")
yr(c(10000, 11000, 12000), "uncal BP")
yr(c(10000, 11000, 12000), "ka")

yr_era() returns details of the era associated with a yr vector:

neolithic <- yr(11700:7500, "BP")
yr_era(neolithic)

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)

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"))

Era definitions {#era}

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")

yr(10000, "BP")

yr_transform(yr(10000, "BP"), "BCE")

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 {#era-list}

all_eras <- eras() 
all_eras$this_year <- NA
na_era <- is.na(era_year_days(all_eras$unit))
all_eras$this_year[!na_era] <- this_year(all_eras$label[!na_era])

knitr::kable(all_eras)

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)

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

Converting between eras: 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")))

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"))

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

yr(500000, "BCE") |> 
  yr_transform(era("mya"), precision = 0.1)

Invalid transformations {#invalidtransform}

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"))
yr_transform(yr(9000, "uncal BP"), era("cal BP"))

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 {#arithmetic}

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

But only when they have the same era:

c <- yr(0.5, "ka")
b - c

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")
era("BP") == era("cal BP")

yr(1000, "BP") + yr(1000, "cal BP")

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

a * b

era documentation built on Nov. 17, 2022, 5:06 p.m.