Number of months between two dates

Question

Is there a standard/common method/formula to calculate the number of months between two dates in R?

I am looking for something that is similar to MathWorks months function

Answer 1

I was about to say that's simple, but difftime() stops at weeks. How odd.

So one possible answer would be to hack something up:

# turn a date into a 'monthnumber' relative to an origin R> monnb <- function(d) { lt <- as.POSIXlt(as.Date(d, origin="1900-01-01")); \                           lt$year*12 + lt$mon }  # compute a month difference as a difference between two monnb's R> mondf <- function(d1, d2) { monnb(d2) - monnb(d1) } # take it for a spin R> mondf(as.Date("2008-01-01"), Sys.Date()) [1] 24 R>  

Seems about right. One could wrap this into some simple class structure. Or leave it as a hack :)

Edit: Also seems to work with your examples from the Mathworks:

R> mondf("2000-05-31", "2000-06-30") [1] 1 R> mondf(c("2002-03-31", "2002-04-30", "2002-05-31"), "2002-06-30") [1] 3 2 1 R>  

Adding the EndOfMonth flag is left as an exercise to the reader :)

Edit 2: Maybe difftime leaves it out as there is no reliable way to express fractional difference which would be consistent with the difftime behavior for other units.