Is there an R package that will handle POSIX objects and return the nth N-day of the week?

I have written a function which, when provided a range of dates, the name of a particular day of the week and the occurrence of that day in a given month (for instance, the second Friday of each month) will return the corresponding date. However, it isn't very fast and I'm not 100% convinced of its robustness开发者_如何转开发. Is there a package or set of functions in R which can do these kinds of operations on POSIX objects? Thanks in advance!

Using the function nextfri whose one line source is shown in the zoo Quick Reference vignette in the zoo package the following gives the second Friday of d where d is the "Date" of the first of the month:

library(zoo)
d <- as.Date(c("2011-09-01", "2011-10-01"))
nextfri(d) + 7
## [1] "2011-09-09" "2011-10-14"

(nextfri is not part of the zoo package -- you need to enter it yourself -- but its only one line)

The following gives the day of the week where 0 is Sunday, 1 is Monday, etc.

as.POSIXlt(d)$wday
## [1] 4 6

If you really are dealing exclusively with dates rather than date-times then you ought to be using "Date" class rather than "POSIXt" classes in order to avoid time zone errors. See the article in R News 4/1.

The timeDate package has some of that functionality; I based this little snippet of code on some code that package. This is for Dates, timeDate has underlying POSIX types.

nthNdayInMonth <- function(date,nday = 1, nth = 1){
  wday <- (as.integer(date) - 3) %% 7
  r <- (as.integer(date) + (nth -1) * 7 + (nday - wday)%%7)
  as.Date(r,"1970-01-01")
}