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

[McGehee, Robert]

Definitely not a bug. You seem to expect seq.Date to have an entirely
different convention depending on whether you are at the beginning of
the month or at the end of the month, which does not seem reasonable to
me.

For instance, does adding a month to Feb 28th give March 28th or March
31st? Given the choices, counting from the beginning of the month seems
much more reasonable. I don't want Feb 1st plus a month to be March 4th!

So if our (extremely reasonable) convention is to count from the
beginning of the month, that is, Feb 28th + 1 month becomes March 28th,
then one would expect August 31 + 1 month to be September 31st. But
since this is not a valid date, seq.Date naturally returns October 1st
(per your example).

To do a sequence along month ends, write a sequence along the beginning
of the month, then subtract 1 day. For instance:

> seq.Date(as.Date("2004-12-31") + 1, as.Date("2005-12-31") + 1, by =
"month") - 1

Alternatively, you should write your own sequence function that defines
month sequences differently for the beginning of the month versus the
end or middle of the month, or in any other way you want (fixed days,
trading days, lunar calendar, etc.).

HTH,
Robert

-----Original Message-----
From: r-devel-bounces at r-project.org
[mailto:r-devel-bounces at r-project.org] On Behalf Of
stephen.ponzio at citigroup.com
Sent: Thursday, August 03, 2006 10:27 AM
To: r-devel at stat.math.ethz.ch
Cc: R-bugs at biostat.ku.dk
Subject: [Rd] seq improperly increments dates (PR#9120)

Full_Name: Stephen Ponzio
Version: 2.3.1
OS: Windows
Submission from: (NULL) (199.67.138.42)


With the option by="1 month" and a date that is the 31st, 
the function seq doesn't give the last day of successive months, 
as I would expect it should:

> Sys.Date()
[1] "2006-08-03"

> Sys.Date()-3
[1] "2006-07-31"

# WORKS OK on Aug. 1:
> seq(Sys.Date()-2, len=5, by="1 month")
[1] "2006-08-01" "2006-09-01" "2006-10-01" "2006-11-01" "2006-12-01"

# DOESN'T WORK on July 31 (list has no date in Sept):
> seq(Sys.Date()-3, len=5, by="1 month")
[1] "2006-07-31" "2006-08-31" "2006-10-01" "2006-10-31" "2006-12-01"

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