Guard against accidental time-zone conversion
> dput(x)
structure(1317830532, class = c("POSIXct", "POSIXt"), tzone = "GMT")
> dput(c(x))
structure(1317830532, class = c("POSIXct", "POSIXt"))
> dput(list(x))
list(structure(1317830532, class = c("POSIXct", "POSIXt"), tzone = "GMT"))
> dput(unlist(list(x)))
1317830532

x <- structure(1317830532, class = c("POSIXct", "POSIXt"),
                 tzone = "GMT")
y <- structure(1317830532+3600, class = c("POSIXct", "POSIXt"),
                 tzone = "PST8PDT")
x
[1] "2011-10-05 16:02:12 GMT"

y
[1] "2011-10-05 10:02:12 PDT"

strftime(c(x, y), format="%Y/%m/%d %H:%M:%S", tz="GMT")
[1] "2011/10/05 16:02:12" "2011/10/05 17:02:12"

strftime(c(x, y), format="%Y/%m/%d %H:%M:%S", tz="PST8PDT")
[1] "2011/10/05 09:02:12" "2011/10/05 10:02:12"

strftime(unlist(y), format="%Y/%m/%d %H:%M:%S", tz="PST8PDT")
[1] "2011/10/05 10:02:12"