[R] Time zones in POSIClt objects

[Jan van der Laan]

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