On Jan 31, 2012, at 2:07 PM, Sarah Goslee wrote:
> I was prompted to try it myself:
>
> On Tue, Jan 31, 2012 at 2:56 PM, Dirk Eddelbuettel <e...@debian.org> wrote:
>>
>> R> seq(as.Date(Sys.Date()), by="-1 months", length=6)
>> [1] "2012-01-31" "2011-12-31" "2011-12-01" "2011-10-31" "2011-10-01"
>> "2011-08-31"
>> R>
>>
>> Notice how October appears twice.
>
> As does December.
>
>> Now, date arithmetic is gruesome but the documentation for seq.Date et al
>> does not hint it wouldn't honour the by= argument. So a bug, or merely a
>> somewhat less than desirable features.
>
> The by argument chokes on "month" if the current day is greater than the
> shortest month in the sequence (presumably due to the irregular nature
> of month lengths):
>
> For leap year 2012:
>> seq(as.Date("2012/1/29"), by="month", length.out=12) # works
> [1] "2012-01-29" "2012-02-29" "2012-03-29" "2012-04-29" "2012-05-29"
> [6] "2012-06-29" "2012-07-29" "2012-08-29" "2012-09-29" "2012-10-29"
> [11] "2012-11-29" "2012-12-29"
>> seq(as.Date("2012/1/30"), by="month", length.out=12) # fails
> [1] "2012-01-30" "2012-03-01" "2012-03-30" "2012-04-30" "2012-05-30"
> [6] "2012-06-30" "2012-07-30" "2012-08-30" "2012-09-30" "2012-10-30"
> [11] "2012-11-30" "2012-12-30"
>
> While for non-leap year 2011:
>> seq(as.Date("2011/1/28"), by="month", length.out=12) # works
> [1] "2011-01-28" "2011-02-28" "2011-03-28" "2011-04-28" "2011-05-28"
> [6] "2011-06-28" "2011-07-28" "2011-08-28" "2011-09-28" "2011-10-28"
> [11] "2011-11-28" "2011-12-28"
>> seq(as.Date("2011/1/29"), by="month", length.out=12) #fails
> [1] "2011-01-29" "2011-03-01" "2011-03-29" "2011-04-29" "2011-05-29"
> [6] "2011-06-29" "2011-07-29" "2011-08-29" "2011-09-29" "2011-10-29"
> [11] "2011-11-29" "2011-12-29"
The issue is the if the next month in sequence does not contain the date, then
the date is advanced until the next valid date. For example:
> seq.Date(as.Date("2012/01/30"), by = "month", length.out = 3)
[1] "2012-01-30" "2012-03-01" "2012-03-30"
February 30th does not exist, thus that date is advanced to March 1st, then the
next date in the sequence is March 30th. Thus, two days in March.
> seq.Date(as.Date("2012/10/31"), by = "month", length.out = 3)
[1] "2012-10-31" "2012-12-01" "2012-12-31"
Here, November 31st does not exist, so the date is advanced to the next valid
date, December 1 and then the next date is December 31. Thus, two days in
December.
So it appears to be working correctly.
HTH,
Marc Schwartz
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