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List:       r-devel
Subject:    [Rd] trunc.Date and round.Date + documentation of DateTimeClasses
From:       SOEIRO Thomas <Thomas.SOEIRO () ap-hm ! fr>
Date:       2021-09-29 14:59:43
Message-ID: 53f342303f1c4fc7b27e4a5f829d80f4 () SCWPR-EXDAG1-6A ! aphm ! ap-hm ! fr
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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

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