Thanks,
On 10/11/24 09:10, Ivan Krylov wrote:
В Thu, 10 Oct 2024 17:16:52 +0200
Jan van der Laan <rh...@eoos.dds.nl> пишет:
This is where it is unclear to me what the purpose is of the `zone`
element of the POSIXlt object. It does allow for registering a time
zone per element. It just seems to be ignored.
I think that since POSIXlt is an interface to what the C standard calls
the "broken-down" time (into parts, not in terms of functionality) and
both the C standard [1] and the POSIX mktime() [2] ignore the
tm_gmtoff/tm_zone fields (standard C because it's not defined there;
POSIX because it defers to standard C), these fields exist for
presentation purposes. They may be populated when constructing the time
object, but not used for later calculations.
Instead, the standard mktime() always uses the process-global timezone,
so when R processes POSIXlt values, it has to set the TZ environment
variable from the 'tzone' attribute, call tzset() to set global state in
the library, use mktime() to obtain seconds since epoch, and then reset
everything back.
So that could then indeed be a performance issue. Still, a warning that
this field is ignored might be nice.
And there are use-cases where it would be nice to have different time
zones per record. For example, for a dataset with flights, departure and
arrival times are often given in the local time zone of the airports and
these local times might be relevant for some analyses. At the same time,
to calculate, for example, flight durations these local time zones need
to be handled correctly.
As I mentioned, fortunately, I only have local time and GMT and it
would be fine to convert them to a single time zone if that is what
it takes to work with them in R.
Since your data looks like the following:
times <- list(
year = c(2024L, 2024L),
month = c(1L, 1L),
day = c(1L, 1L),
hour = c(12L, 12L),
minutes = c(30L, 30L),
seconds = c(0, 0),
timezone = c("", "GMT")
)
how about converting all the times into POSIXct?
do.call(mapply, c(
\(year, month, day, hour, minutes, seconds, timezone)
"%04d-%02d-%02d %02d:%02d:%02d" |>
sprintf(year, month, day, hour, minutes, seconds) |>
as.POSIXct(format = "%Y-%m-%d %H:%M:%S", tz = timezone),
times
)) |> do.call(c, args=_)
Its 'tzone' attribute exists mostly for presentation purposes, so even
if you lose it, the exact point in UTC-relative time is still intact.
Thanks, I had already started to do something similar starting from
POSIXlt: a funtion that converts to POSIXct using the zone information:
lttoct <- function(x) {
tzone <- attr(x, "tzone")[1]
result <- rep(as.POSIXct(0, tz = tzone), length(res))
zones <- unique(x$zone)
for (zone in zones) {
sel <- if (is.na(zone)) is.na(x$zone) else
x$zone == zone & !is.na(x$zone)
result[sel] <- as.POSIXct(x[sel], tz = zone)
}
result
}
t1 <- as.POSIXlt(c("2023-01-01 12:30", "2024-01-01 12:30"))
t1$zone <- c("", "GMT")
lttoct(t1)
# [1] "2023-01-01 12:30:00 CET" "2024-01-01 13:30:00 CET"
as.POSIXct(t1)
# [1] "2023-01-01 12:30:00 CET" "2024-01-01 12:30:00 CET"
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