leap-years.r

#' Is a year a leap year?
#'
#' If x is a recognized date-time object, leap_year will return whether x 
#' occurs during a leap year. If x is a number, leap_year returns whether it 
#' would be a leap year under the Gregorian calendar. 
#'
#' @export leap_year
#' @param date a date-time object or a year 
#' @return TRUE if x is a leap year, FALSE otherwise
#' @keywords logic chron
#' @examples
#' x <- as.Date("2009-08-02")
#' leap_year(x) # FALSE
#' leap_year(2009) # FALSE
#' leap_year(2008) # TRUE
#' leap_year(1900) # FALSE
#' leap_year(2000) # TRUE
leap_year <- function(date) {
  recognized <- recognize(date)
  if (recognized)
    year <- year(date)
  else if (all(is.numeric(date)))
    year <- date
  else
    stop("unrecognized date format")
  (year %% 4 == 0) & ((year %% 100 != 0) | (year %% 400 == 0))
}