[R] POSIXct time zone and daylight savings issues

[Sebastian P. Luque]

Hello,

Suppose we need a function that takes a POSIXct object and need to
calculate the time difference between it and GMT time:


gmtDiff <- function(time) {
    time.gmt <- as.POSIXct(format(time, tz="GMT"))
    time.plt <- as.POSIXlt(time)
    dlstime <- ifelse(time.plt$isdst > 0, 1, 0)
    timezone <- as.numeric(difftime(time, time.gmt, units="hours"))
    timezone - dlstime
}


Please note this assumes no daylight savings if the 'isdst' component for
the POSIXlt object is negative (i.e. don't know if DST is in effect).  Now
if we pass some values to this function:


R> gmtDiff(as.POSIXct("2006-10-27", tz="GMT"))
[1] -5
R> gmtDiff(as.POSIXct(Sys.time()))
[1] -6


I would have expected the first call to return zero, but it seems as if
the calculation is always done with respect to the system's locale, and I
can't understand what the function of the 'tzone' attribute is in these
calculations.  ?strptime has information on how to change the locale for
some operations, which in this case didn't change the outcome:


R> lct <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
[1] "C"
R> gmtDiff(as.POSIXct("2006-10-27", tz="GMT"))
[1] -5
R> gmtDiff(as.POSIXct(Sys.time()))
[1] -6
R> Sys.setlocale("LC_TIME", lct)
[1] "en_CA.UTF-8"
R> sessionInfo()
R version 2.4.0 (2006-10-03) 
x86_64-pc-linux-gnu 

locale:
LC_CTYPE=en_CA.UTF-8;LC_NUMERIC=C;LC_TIME=en_CA.UTF-8;LC_COLLATE=en_CA.UTF-8;LC_MONETARY=en_CA.UTF-8;LC_MESSAGES=en_CA.UTF-8;LC_PAPER=en_CA.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_CA.UTF-8;LC_IDENTIFICATION=C

attached base packages:
[1] "methods"   "stats"     "graphics"  "grDevices" "utils"     "datasets" 
[7] "base"     

other attached packages:
  lattice 
 "0.14-9" 


I'd be grateful for any clarification and advice on how to properly
calculate this time difference.


Cheers,

-- 
Seb