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GH-35633: [R] R builds failing with error 'Invalid: Timestamps already have a timezone: 'UTC'. Cannot localize to 'UTC'' #35671

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May 18, 2023
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GH-35633: [R] R builds failing with error 'Invalid: Timestamps already have a timezone: 'UTC'. Cannot localize to 'UTC'' #35671

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8 changes: 5 additions & 3 deletions r/R/dplyr-funcs-datetime.R
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Original file line number Diff line number Diff line change
Expand Up @@ -61,12 +61,14 @@ register_bindings_datetime_utility <- function() {
tz <- Sys.timezone()
}

# if a timestamp does not contain timezone information (i.e. it is
# If a timestamp does not contain timezone information (i.e. it is
# "timezone-naive") we can attach timezone information (i.e. convert it into
# a "timezone-aware" timestamp) with `assume_timezone`
# if we want to cast to a different timezone, we can only do it for
# timezone-aware timestamps, not for timezone-naive ones
if (!is.null(tz)) {
# timezone-aware timestamps, not for timezone-naive ones.
# strptime in Acero will return a timezone-aware timestamp if %z is
# part of the format string.
if (!is.null(tz) && !grepl("%z", format, fixed = TRUE)) {
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I don't understand how timezones work in R (and with POSIXlt and POSIXct), but with the above change, the tz keyword will just be ignored when the string includes an offset?

That doesn't match fully what base R does:

> strptime("2020-05-01T00:00+0100", format="%Y-%m-%dT%H:%M%z")
[1] "2020-05-01 01:00:00"
> strptime("2020-05-01T00:00+0100", format="%Y-%m-%dT%H:%M%z", tz="US/Eastern")
[1] "2020-04-30 19:00:00"

(but again, I don't really know how to interpret those returned values, and whether they are tz "aware" or "native" or even if those concepts even exist in R. I looked at the $zone and $gmtoff attributes of the above return values, but can't make sense of it)

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We don't have a naive datetime type in R...everything is timezone aware, and tz = "" means "your local timezone" (but everything is UTC internally). What you've encountered here is a fun type called the POSIXlt which is sort of like a data frame that stores components in separate vectors (the POSIXct is a more normal version of it, which is seconds from the unix epoch as a double vector).

It's a good point that this doesn't match what base R does...I think we'd want to cast() if grepl("%z") and assume_timezone() otherwise?

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I don't know if this helps with understanding dates/times in R, but the tz argument won't affect the underlying point in time (just the attribute of the output and, to great confusion of many, how it is printed):

unclass(as.POSIXct(strptime("2020-05-01T00:00+0100", format="%Y-%m-%dT%H:%M%z")))
#> [1] 1588287600
#> attr(,"tzone")
#> [1] ""
unclass(as.POSIXct(strptime("2020-05-01T00:00+0100", tz = "UTC", format="%Y-%m-%dT%H:%M%z")))
#> [1] 1588287600
#> attr(,"tzone")
#> [1] "UTC"
unclass(as.POSIXct(strptime("2020-05-01T00:00+0100", tz = "America/Halifax", format="%Y-%m-%dT%H:%M%z")))
#> [1] 1588287600
#> attr(,"tzone")
#> [1] "America/Halifax"

Created on 2023-05-24 with reprex v2.0.2

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It's a good point that this doesn't match what base R does...I think we'd want to cast() if grepl("%z") and assume_timezone() otherwise?

Yes, that sounds correct

output <- Expression$create(
"assume_timezone",
output,
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