r - Subtract exactly one year from a POSIXct object

Question

lets say we have this date "2014-05-11 14:45:00 UTC". I would like to get the exact POSIXct object for 1 year before so "2013-05-11 14:45:00 UTC".

My first thought is to create a whole new POSIXct object by subtracting one from the year bit and pasting it together with the remainder of the string and then creating a new POSIXct object with that string like so:

time <- as.POSIXct("2014-05-11 14:45:00 UTC",tz="UTC",origin="1970-01-01")
newTime <- as.POSIXct(paste(as.character(as.numeric(substr(time,1,4)) - 1),substr(time,5,19),sep=""),tz="UTC",origin="1970-01-01")

this works fine (except in case of leap years!) but the thing is I need to do this in a large data.table for each row and preferably put the results right back in data.table. Is there any other way of subtracting a year off an object like this?

Some extra I need to apply this to a data.table like this one:

          Time
 1: 1349206200
 2: 1349207100
 3: 1349208000
 4: 1349208900
 5: 1349209800
 6: 1349210700
 7: 1349211600
 8: 1349212500
 9: 1349213400
10: 1349214300
11: 1349215200

but this happens when I do:

SOdata[,Time:=as.numeric(as.POSIXct(paste(as.character(as.numeric(substr(Time,1,4)) - 1),substr(Time,5,19),sep=""),tz="UTC",origin="1970-01-01"))]
Error in as.POSIXlt.character(x, tz, ...) : 
  character string is not in a standard unambiguous format

I am guessing I need to use something like lapply, but I always mess up syntax when using that function. So does anyone know how?

Answer 1

lubridate is your friend.

library(lubridate)
time <- as.POSIXct("2014-05-11 14:45:00 UTC",tz="UTC",origin="1970-01-01")
time-dyears(1)
#[1] "2013-05-11 14:45:00 UTC"
time+dyears(1)
#[1] "2015-05-11 14:45:00 UTC"

For leap years

> x <- as.POSIXct(c("2012-02-28", "2012-02-29"), tz="UTC",origin="1970-01-01")
> x - dyears(1)
[1] "2011-02-28 UTC" "2011-03-01 UTC"