[R] POSIXct time zone and daylight savings issues

[Gabor Grothendieck]

Try this:

gmtDiff <- function(time) time - as.POSIXct(format(time), tz = "GMT")

gmtDiff(Sys.time())
gmtDiff(as.POSIXct("2006-10-27", tz = "GMT"))

which both give me the correct answer currently.

The expression after the minus sign comes from the table at the end
of the help desk article in R News 4/1.  I think you got the parentheses
wrong in your version and I think the isdst calculation needs to
be omitted -- I get the right answer if I leave it out but wrong with it.

On 10/27/06, Sebastian P. Luque <spluque at gmail.com> wrote:
> 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
>
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