[Rd] trunc.Date and round.Date + documentation of DateTimeClasses

[Martin Maechler]

Excuse the exceptional top-reply:

Note that a very related issue has been raised not so long ago
by Dirk (in CC) on R's Bugzilla :

  trunc.Date should support months and years arguments as trunc.POSIXt does 
  https://bugs.r-project.org/show_bug.cgi?id=18099

which had some agreement (also with you: I agree we should
change something about this) but I also had proposed to approach
it more generally than in the PR .. which you already did by
mentioning trunc() and round() methods together.

Still, Dirk's proposal would try harder to remain back
compatible in those cases where  trunc.Date() currently does
"behave as it should".

Martin Maechler
ETH Zurich  and  R Core

>>>>> SOEIRO Thomas 
>>>>>     on Thu, 30 Sep 2021 10:32:32 +0000 writes:

    > About fractional days, trunc.Date2 actually seems to have no regression and to be backward compatible compared to the original trunc.Date:

    > frac <- as.Date("2020-01-01") + 0.5
    > identical(trunc(frac), trunc.Date2(frac))

    > (I may still miss something since I do not understand how
    > trunc.Date manage fractional days with round(x - 0.4999999).)

    > -----Message d'origine-----
    > De : SOEIRO Thomas 
    > Envoyé : mercredi 29 septembre 2021 17:00
    > À : 'r-devel using r-project.org'
    > Objet : trunc.Date and round.Date + documentation of DateTimeClasses

    > Dear All,

    > 1) trunc.Date and round.Date:

    > Currently, the help page for trunc.Date and round.Date
    > says "The methods for class "Date" are of little use
    > except to remove fractional days". However, e.g.,
    > trunc.POSIXt(Sys.Date(), "years") and
    > round.POSIXt(Sys.Date(), "years") work because the
    > functions start with x <- as.POSIXlt(x).

    > Would you consider a simple implementation of trunc.Date
    > and round.Date based on trunc.POSIXt and round.POSIXt?
    > This would enable to avoid coercion from Date to POSIXt
    > and back to Date for these simple manipulations.

    > For example:
    > # (I do not have a clear understanding of what "remove fractional days" means, and I did not implement it.)
	
> trunc.Date2 <-
>   function(x, units = c("days", "months", "years"), ...)
>   {
>     units <- match.arg(units)
>     x <- as.POSIXlt(x)
>     
>     switch(units,
>            "days" = {
>              x$sec[] <- 0; x$min[] <- 0L; x$hour[] <- 0L;
>              x$isdst[] <- -1L
>            },
>            "months" = {
>              x$sec[] <- 0; x$min[] <- 0L; x$hour[] <- 0L;
>              x$mday[] <- 1L
>              x$isdst[] <- -1L
>            },
>            "years" = {
>              x$sec[] <- 0; x$min[] <- 0L; x$hour[] <- 0L;
>              x$mday[] <- 1L; x$mon[] <- 0L
>              x$isdst[] <- -1L
>            }
>     )
>     as.Date(x)
>   }



    > 2) documentation of DateTimeClasses:

    > It may be useful to add in the documentation of
    > DateTimeClasses that manipulating elements of POSIXlt
    > objects may results in "invalid" entries (e.g., mon = 12
    > or mday = 0), but that the object is nevertheless
    > correctly printed/coerced.

    > Is this behavior explicitly supported?

    > d <- as.POSIXlt("2000-01-01")
    > unclass(d)
    > d$mon <- d$mon + 12
    > d$mday <- d$ mday - 1
    > unclass(d)
    > d
    > d <- as.POSIXlt(as.POSIXct(d))
    > dput(d)



    > Best,
    > Thomas