R devel - Sep 2021 - 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