Problems adding a month to X using POSIXlt in R - need to reset value using as.Date(X)

Question

This works for me in R:

# Setting up the first inner while-loop controller, the start of the next water year
  NextH2OYear <- as.POSIXlt(firstDate)
  Next         


        

Answer 1

Here is you can add 1 month to a date in R, using package lubridate:

library(lubridate)
x <- as.POSIXlt("2010-01-31 01:00:00")
month(x) <- month(x) + 1

>x
[1] "2010-03-03 01:00:00 PST"

(note that it processed the addition correctly, as 31st of Feb doesn't exist).

Answer 2

Plain R

seq(Sys.Date(), length = 2, by = "month")[2]
seq(Sys.Date(), length = 2, by = "year")[2]

Note that this works with POSIXlt too, e.g.

seq(as.POSIXlt(Sys.Date()), length = 2, by = "month")[2]

mondate.

library(mondate)
now <- mondate(Sys.Date())
now + 1  # date in one month
now + 12  # date in 12 months

Mondate is bit smarter about things like mondate("2013-01-31")+ 1 which gives last day of February whereas seq(as.Date("2013-01-31"), length = 2, by = "month")[2] gives March 3rd.

yearmon If you don't really need the day part then yearmon may be preferable:

library(zoo)
now.ym <- yearmon(Sys.Date())
now.ym + 1/12 # add one month
now.ym + 1 # add one year

ADDED comment on POSIXlt and section on yearmon.

Answer 3

Can you perhaps provide a reproducible example? What's in firstDate, and what version of R are you using? I do this kind of manipulation of POSIXlt dates quite often and it seems to work:

Sys.Date()
# [1] "2013-02-13"
date = as.POSIXlt(Sys.Date())
date$mon = date$mon + 1
as.Date(date)
# [1] "2013-03-13"