[GitHub] [arrow] djnavarro edited a comment on pull request #12154: ARROW-14821: [R] Implement bindings for lubridate's floor_date, ceiling_date, and round_date

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djnavarro edited a comment on pull request #12154:
URL: https://github.com/apache/arrow/pull/12154#issuecomment-1063692453


   Maybe it's easiest to illustrate with actual R code. The behaviour I need to emulate with regards to the `change_on_boundary` argument looks different depending on whether the data are stored as a datetime (e.g. POSIXct in R, timestamp in Arrow) versus a Date (e.g., Date in R, date32 in Arrow):
   
   ```r
   # a datetime object at the midnight boundary, and a date object with no time
   midnight_time <- as.POSIXct(strptime("2022-03-10 00:00:00", format = "%F %T"))
   timeless_date <- as.Date("2022-03-10")
   
   # ceiling date applied to the datetime
   ceiling_date(midnight_time, unit = "day", change_on_boundary = NULL)  # returns the 10th
   ceiling_date(midnight_time, unit = "day", change_on_boundary = TRUE)  # returns the 11th
   ceiling_date(midnight_time, unit = "day", change_on_boundary = FALSE) # returns the 10th
   
   # ceiling date applied to the date
   ceiling_date(timeless_date, unit = "day", change_on_boundary = NULL)  # returns the 11th
   ceiling_date(timeless_date, unit = "day", change_on_boundary = TRUE)  # returns the 11th
   ceiling_date(timeless_date, unit = "day", change_on_boundary = FALSE) # returns the 10th
   ```
   
   This pattern only affects times and dates deemed to be on the boundary with respect to the rounding unit. Any time past midnight and `lubridate::ceiling_date()` would always return the 11th for the datetime object. 
   
   Currently, the `ceil_temporal` C++ function would return the 10th for the analogous cases. ~~If I can work out how to add +1 time unit to the boundary cases then I have no problems exactly mimicking lubridate behaviour. I can get this to work for timestamp objects. This code works, for instance, and adds 1 day to the Arrow timestamp that corresponds to `midnight_time`:~~
   
   ```r
   call_function("add_checked", Scalar$create(midnight_time), Scalar$create(as.difftime("24:00:00")))
   ```
   
   ~~But when I try the same thing for the date, however, it throws this error:~~
   
   ```
   Error: NotImplemented: Function 'add_checked' has no kernel matching input types (scalar[date32[day]], scalar[duration[s]])
   ```
   
   ~~That seems odd to me and I'm not sure why I'm getting that error because I'm pretty sure you implemented this in [ARROW-14901](https://github.com/apache/arrow/pull/12377). So I must be doing something wrong?~~
   
   EDIT: Never mind, I worked it out. 🤦🏻‍♀️  I can handle the `change_on_boundary` behaviour on the R side! 
   
   


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