R/get_age.R

In PAutilities: Streamline Physical Activity Research

#' Calculate age
#'
#' Takes two Date objects and calculates age based on
#' \code{\link[base]{difftime}} (in days) divided by 365.2425 days per year (for
#' age in years) or 30.4375 days per month (for age in months).
#'
#' @param birthdate Date object giving the date of birth
#' @param current_date Date object giving the date from which age is to be
#'   calculated
#' @param units The units in which age should be reported
#'
#' @return Numeric value giving age in the specified units.
#' @export
#'
#' @examples
#' get_age(as.Date("2000-01-01"), Sys.Date(), "years")
get_age <- function(
  birthdate, current_date, units = c("years", "months")
) {

  stopifnot(
    inherits(birthdate, "Date"),
    inherits(current_date, "Date")
  )

  units <- match.arg(
    units, c("years", "months"), several.ok = TRUE
  )

  age_days <-
    difftime(
      current_date, birthdate, units = "days"
    ) %>%
    as.numeric(.)

  if ("years" %in% units) {
    years <- age_days/365.2425
    if (length(units) == 1) return(years)
  }

  if ("months" %in% units) {
    months <- age_days/30.4375
    if (length(units) == 1) return(months)
  }

  data.frame(years = years, months = months)

}