[Rd] seq improperly increments dates (PR#9120)

[Gabor Grothendieck]

Here are some more seq.dates examples from chron.
It seems that if seq.Date cannot output day of the month n
(because the month has fewer days) then it outputs n days from
the start of the month even if that goes into the next month
whereas seq.dates in chron outputs the end of the month in that case.

> library(chron)
> seq(chron("01/31/2006"), by = "month", length = 6)
[1] 01/31/06 02/28/06 03/31/06 04/30/06 05/31/06 06/30/06

> seq(chron("01/30/2006"), by = "month", length = 6)
[1] 01/30/06 02/28/06 03/30/06 04/30/06 05/30/06 06/30/06

> seq(chron("01/29/2006"), by = "month", length = 6)
[1] 01/29/06 02/28/06 03/29/06 04/29/06 05/29/06 06/29/06

> seq(chron("01/28/2006"), by = "month", length = 6)
[1] 01/28/06 02/28/06 03/28/06 04/28/06 05/28/06 06/28/06

> seq(chron("01/27/2006"), by = "month", length = 6)
[1] 01/27/06 02/27/06 03/27/06 04/27/06 05/27/06 06/27/06


On 8/4/06, Martin Maechler <maechler at stat.math.ethz.ch> wrote:
> >>>>> "Gabor" == Gabor Grothendieck <ggrothendieck at gmail.com>
> >>>>>     on Thu, 3 Aug 2006 20:14:24 -0400 writes:
>
>    Gabor> That's, in fact, the way seq.dates works in the chron package:
>    Gabor> library(chron)
>    Gabor> x <- chron("01/31/2006")
>    Gabor> seq(x, by = "month", length = 2)   #  01/31/06 02/28/06
>
> Hmm, so, by "logic",
> 2006-01-31 + 1month  |->  2006-02-28
> 2006-01-30 + 1month  |->  2006-02-27 (?)
> 2006-01-29 + 1month  |->  2006-02-26 (?)
> 2006-01-28 + 1month  |->  2006-02-25  ????????
>
> I really don't like 'chron's behavior which seems much less
> logical to me than what R does with the official "Date" objects :
>
> > options(width=88)
> > for(d in 28:31) print(seq(as.Date(paste("2006-01", d, sep="-")), len = 6, by="1 month"))
> [1] "2006-01-28" "2006-02-28" "2006-03-28" "2006-04-28" "2006-05-28" "2006-06-28"
> [1] "2006-01-29" "2006-03-01" "2006-03-29" "2006-04-29" "2006-05-29" "2006-06-29"
> [1] "2006-01-30" "2006-03-02" "2006-03-30" "2006-04-30" "2006-05-30" "2006-06-30"
> [1] "2006-01-31" "2006-03-03" "2006-03-31" "2006-05-01" "2006-05-31" "2006-07-01"
> >
>
> {which, BTW, *is* reproducible code; the bug report was only
>  reproducible on the day it was posted because it sillily used
>  Sys.date()}
>
>    Gabor> See the help desk article in R News 4/1 for more about the main
>    Gabor> date classes.
>
>    Gabor> On 8/3/06, Ponzio, Stephen [CIB-LAVA] <stephen.ponzio at citigroup.com> wrote:
>    >> You're right, it's tricky.
>    >>
>    >> I guess I would expect Jan. 30 + 1 month = Feb. 28.
>    >>
>    >> Of couse, then  Jan. 30 + 1 month = Jan. 28 + 1 month;
>    >> I understand.
>    >>
>    >> Being that "1 month" is imprecise in terms of number of days,
>    >> this anomaly is preferable to skipping months (Jan. 30 + 1 month = March 2),
>    >> in my opinion.
>    >>
>    >> That is what I expected, anyway.
>    >>
>    >> Thanks,
>    >> Stephen
>
>   [.........]
>