[GitHub] [arrow] jorisvandenbossche commented on a diff in pull request #35671: GH-35633: [R] R builds failing with error 'Invalid: Timestamps already have a timezone: 'UTC'. Cannot localize to 'UTC''

["jorisvandenbossche (via GitHub)" <gi...@apache.org>]

jorisvandenbossche commented on code in PR #35671:
URL: https://github.com/apache/arrow/pull/35671#discussion_r1203809343


##########
r/R/dplyr-funcs-datetime.R:
##########
@@ -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)) {

Review Comment:
   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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