[Rd] A potential POSIXlt->Date bug introduced in r-devel

[Martin Maechler]

>>>>> Davis Vaughan 
>>>>>     on Wed, 5 Oct 2022 17:04:11 -0400 writes:

    > Hi all,

    > I think I have discovered a bug in the conversion from POSIXlt to Date that
    > has been introduced in r-devel.

    > It affects lubridate, but surprisingly didn't cause test failures there.
    > Instead it caused test failures in users of lubridate, like slider, arrow,
    > and admiral (see https://github.com/tidyverse/lubridate/issues/1069), and
    > at least in slider I have been asked by CRAN to correct this issue before
    > 2022-10-16.

    > In r-devel we get the following:

    > ```
    > data <- list(
    > sec = 0,
    > min = 0L,
    > hour = 0L,
    > mday = 31L,
    > mon = c(0L, NA, 2L),
    > year = 113L,
    > wday = 4L,
    > yday = 30L,
    > isdst = 0L
    > )

    > x <- .POSIXlt(xx = data, tz = "UTC")
    > x
    > #> [1] "2013-01-31 UTC" NA               "2013-03-31 UTC"

    > # Looks right
    > as.POSIXct(x)
    > #> [1] "2013-01-31 UTC" NA               "2013-03-31 UTC"



    > # Weird, where is the `NA`?
    > as.Date(x)
    > #> [1] "2013-01-31" "1970-01-01" "2013-03-31"
    > ```

I agree that the above is wrong, i.e., a bug in current  R-devel.

    > The POSIXlt object is length 3, but is only partially filled out. 

    > The other elements are all recycled to length 3 upon
    > conversion to POSIXct or Date. 

    > But when converting to Date, we lose the `NA` value. I think the
    > `as.Date()` conversion seems inconsistent with the `as.POSIXct()`
    > conversion.

Yes.  There was another very much relatd conversation here on R-devel,
initiated by Suharto Anggono just a few days ago.

This subject, i.e., "partially filled out" POSIXlt objects, was
one of the topics, too.

See my reply there, notably at the end:

  https://stat.ethz.ch/pipermail/r-devel/2022-October/082072.html
    
I do mention that "recycling" of partially filled POSIXlt
objects has only partially been implemented in R more generally
and was actually asking for comments and further discussion.


    > It looks like this comes up because the conversion to Date now defaults to
    > using `sec` if any of the date-like fields are `NA_INTEGER`,

yes, because only that allows to also deal with +/- Inf  etc,
as was recently added as new feature, see the NEWS of R 4.2.0

    • Not strictly fixing a bug, format()ing and print()ing of
      non-finite Date and POSIXt values NaN and +/-Inf no longer show
      as NA but the respective string, e.g., Inf, for consistency with
      numeric vector's behaviour, fulfilling the wish of PR#18308.

i.e., see also R's bugzilla
      https://bugs.r-project.org/show_bug.cgi?id=18308

which actually *also* mentioned an NA problem in Date/Time objects.



    >  but this means  the `NA` in the `mon` field is ignored.

which I agree is bogous and we'll fix.

Still, I did not get any feedback on asking about documentation
etc on  POSIXlt objects ... and I *had* mentioned I agreed that
the current partial implementation of  "partially filled" i.e. recycling of
POSIXlt components should probably be made part of the
"definition" of POSIXlt.

Have I overlooked an existing definition / contract about these?

Martin

--
Martin Mächler
ETH Zurich   and  R Core team