Thanks. On 10/10/24 16:13, Jeff Newmiller wrote:
POSIXt vectors do not support different time zones element-to-element.
> I complained about this on this list a couple of decades ago, and was chastised for it. Evidently handling timezones per element was considered to be too impractically slow to be a standard feature.
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.
If you want to keep track of timezones per element, you have to create a vector of timestamps (I would recommend POSIXct using UTC) and a parallel vector of timezone strings. How you manipulate these depends on your use cases, but from R's perspective you will have to manipulate them element-by-element.
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. So, I guess, I could split the vector in two, convert local time to GMT and combine them again (respecting the original order).
Jan
On October 10, 2024 6:46:19 AM PDT, Jan van der Laan <rh...@eoos.dds.nl> wrote:It is not completely clear to me how time zones work with POSIXlt objects. For POSIXct, I can understand what happens: time is always stored in GMT, the `tzone` attribute only affects how the times are displayed. All computations etc. are done in GMT. POSIXlt objects have both a `tzone` attribute and a `zone` field. It seems that the `zone` field is largely ignored. It only seems to be used for displaying the times, but does not seem to play a role when doing arithmetic and conversions of the times. For example below, we have the same times in two different time zones. The following seems to do what I expect: when we subtract the two times we get the difference in time between the two time zones: t1 <- as.POSIXlt(c("2024-01-01 12:30", "2024-01-01 12:30"), tz = "GMT") t1$zone # [1] "GMT" "GMT" t2 <- as.POSIXlt(c("2024-01-01 12:30", "2024-01-01 12:30")) t2$zone # [1] "CET" "CET" t1 - t2 # Time differences in hours # [1] 1 1 When I change the `tzone` attribute of t1 to that of t2: attr(t1, "tzone") <- attr(t2, "tzone") t1 #[1] "2024-01-01 12:30:00 GMT" "2024-01-01 12:30:00 GMT" The times are still displayed as being in GMT, however when I take the difference: t1 - t2 #Time differences in secs #[1] 0 0 We get a difference of 0. So it seems that the difference is only based on the `tzone` attribute. The value of `zone` is completely ignored. I am aware of the following remark in ?POSIXlt on arithmetic operations | Be aware that ‘"POSIXlt"’ objects will be interpreted as being in | the current time zone for these operations unless a time zone has | been specified. but this does not explain this, I think. One of the reasons, I ask, is that I have (potentially) times in different time zones. Using POXIXlt objects seems like they could store/support this. But working with this seems unpractical as the `zone` field does not seem to do anything: t1$zone <- c("CET", "GMT") t1 - t2 #Time differences in secs #[1] 0 0 Also the `gmtoff` field does not seem to do anything. For what/where is this field used? t1$gmtoff <- c(3600, 0) t1 #[1] "2024-01-01 12:30:00 CET" "2024-01-01 12:30:00 GMT" t1 - t2 #Time differences in secs #[1] 0 0 as.POSIXct(t1) #[1] "2024-01-01 12:30:00 CET" "2024-01-01 12:30:00 CET" So, I am not sure what purpose the zone and gmtoff fields have. Do they have a purpose? Am I using them wrong? The reason I am asking, is that I have some times in potentially different time zones. The data I get is something like: 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") ) I am looking for ways to convert this into a practical date format for working with in R. Possible time zones are only local time or UTC/GMT. I would be fine with either converting local time to GMT. What would be a good way to convert these to a format R can work with? Thanks for the help. Jan ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide https://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
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