Elapsed Months in `R`

elapsedMonths.R

# A simple function to compute elapsed months between start date and end date

elapsed_months <- function(end_date, start_date) {
    ed <- as.POSIXlt(end_date)
    sd <- as.POSIXlt(start_date)
    12 * (ed$year - sd$year) + (ed$mon - sd$mon)
}

#
# Example:
#
#  >Sys.time()
#  [1] "2014-10-29 15:45:44 CDT"
#  >elapsed_months(Sys.time(), as.Date("2012-07-15"))
#  [1] 27
#  >elapsed_months("2002-06-30", c("2002-03-31", "2002-04-30", "2002-05-31"))
#  [1] 3 2 1
#  To me it makes sense to think about this problem as simply subtracting two dates, and since minuend − subtrahend = # difference (wikipedia), I put the later date first in the parameter list.
#  Note that it works fine for dates preceeding 1900 despite those dates having internal representations of year as negative, thanks to the rules for subtracting negative numbers...
#