Subtracting months - issue with last day of month?

Question

Quick question on dates in R. Check out this snippet of code:

Sys.Date() - months(3)
# [1] \"2013-12-31\"
Sys.Date() - months(18)
# [1] NA

Answer 1

The lubridate functions %m+% and %m-% are designed to handle this issue ("Add and subtract months to a date without exceeding the last day of the new month").

library(lubridate)
Sys.Date() %m-% months(18)
# [1] "2012-09-30"

# or to make it reproducible if Sys.Date() happens to be different from that in OP
as.Date("2014-03-31") %m-% months(18)
# [1] "2012-09-30"

# example of %m+%
as.Date("2014-01-31") + months(1)
# [1] NA
as.Date("2014-01-31") %m+% months(1)
# [1] "2014-02-28"