Time zones in POSIClt objects

?s 15:13 de 10/10/2024, Jeff Newmiller via R-help escreveu:
POSIXt vectors do not support different time zones element-to-element.

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.

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.

On October 10, 2024 6:46:19 AM PDT, Jan van der Laan <rhelp at 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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Hello,

A way to have different time zones is to store t1 and t2 in list, which 
are vectors. Just not atomic vectors.
I think it complicates what should be simple, but here it is.

# create two lists
t1 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz = 
"GMT")
t2 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz = 
"CET")

# this works but is it a wanted way of making simple computations?
Map(`-`, t1, t2)
#> [[1]]
#> Time difference of 1 hours
#>
#> [[2]]
#> Time difference of 1 hours

# mapply default is to simplify the result, if possible
mapply(`-`, t1, t2)
#> [1] 1 1

t1 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz = 
"GMT")
t2 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz = 
"CET")

# as documented in ?mapply > sapply, all attributes are lost,
# after simplification the class attribute follows the hierarchy
# NULL < raw < logical < integer < double < complex < character < list < 
expression
mapply(`-`, t1, t2) |> str()
#>  num [1:2] 1 1

# now change only one member of the list t1
attr(t1[[2]], "tzone") <- attr(t2[[2]], "tzone")

# t1 has two different time zones and the Map/mapply loops
# above still give the expected results
Map(`-`, t1, t2)
#> [[1]]
#> Time difference of 1 hours
#>
#> [[2]]
#> Time difference of 0 secs

mapply(`-`, t1, t2)
#> [1] 1 0

Hope this helps,

Rui Barradas
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Time zones in POSIClt objects

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