[GitHub] [arrow] paleolimbot 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''

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

paleolimbot commented on code in PR #35671:
URL: https://github.com/apache/arrow/pull/35671#discussion_r1204045935


##########
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:
   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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