Adjust data time zone in R - r

Question

For some reason I can't adjust the time zone by as.POSIXlt.
time <- "Wed Jun 22 01:53:56 +0000 2016"
t <- strptime(time, format = '%a %b %d %H:%M:%S %z %Y')
t
[1] "2016-06-21 21:53:56"
Can't change the time zone
as.POSIXlt(t, "EST")
[1] "2016-06-21 21:53:56"
as.POSIXlt(t, "Australia/Darwin")
[1] "2016-06-21 21:53:56"
Can change the time zone for Sys.time()
as.POSIXlt(Sys.time(), "EST")
[1] "2016-09-26 01:47:22 EST"
as.POSIXlt(Sys.time(), "Australia/Darwin")
[1] "2016-09-26 16:19:48 ACST"
How to solve it?

Answer 1

Try this:
time <- "Wed Jun 22 01:53:56 +0000 2016"
strptime(time, format = '%a %b %d %H:%M:%S %z %Y')
#[1] "2016-06-22 07:23:56"
strptime(time, format = '%a %b %d %H:%M:%S %z %Y', tz="EST")
#[1] "2016-06-21 20:53:56"
strptime(time, format = '%a %b %d %H:%M:%S %z %Y', tz="Australia/Darwin")
#[1] "2016-06-22 11:23:56"

Answer 2

strptime returns a POSIXlt object. Calling as.POSIXlt on t just returns t. There is no as.POSIXlt.POSIXlt method, so as.POSIXlt.default is dispatched. And you can see the first if statement checks if x inherits the POSIXlt class, and returns x if that's true.
str(t)
# POSIXlt[1:1], format: "2016-06-21 20:53:56"
print(as.POSIXlt.default)
# function (x, tz = "", ...)
# {
# if (inherits(x, "POSIXlt"))
# return(x)
# if (is.logical(x) && all(is.na(x)))
# return(as.POSIXlt(as.POSIXct.default(x), tz = tz))
# stop(gettextf("do not know how to convert '%s' to class %s",
# deparse(substitute(x)), dQuote("POSIXlt")), domain = NA)
# }
# <bytecode: 0x2d6aa18>
# <environment: namespace:base>
You either need to use as.POSIXct instead of strptime and specify the timezone you want, then convert to POSIXlt:
ct <- as.POSIXct(time, tz = "Australia/Darwin", format = "%a %b %d %H:%M:%S %z %Y")
t <- as.POSIXlt(ct)
Or use strptime and convert t to POSIXct and then back to POSIXlt:
t <- strptime(time, format = "%a %b %d %H:%M:%S %z %Y")
t <- as.POSIXlt(as.POSIXct(t, tz = "Australia/Darwin"))