[R] Time zones in POSIClt objects

[Jeff Newmiller]

I am not sure what this has to do with timezones embedded in specific POSIXt vectors? Can you elaborate why this is relevant?

On October 10, 2024 11:32:31 AM PDT, Gabor Grothendieck <ggrothendieck using gmail.com> wrote:
>Sys.setenv(TZ = "GMT") will set the local time zone to GMT so there
>would only be one time
>zone regardless of whether local or GMT were used.
>
>On Thu, Oct 10, 2024 at 11:17 AM Jan van der Laan <rhelp using eoos.dds.nl> wrote:
>>
>> 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 <rhelp using 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
>> >>
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>> >
>>
>> ______________________________________________
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>> and provide commented, minimal, self-contained, reproducible code.
>
>
>

-- 
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