[R] Looking for a special date function in R

[Prof Brian Ripley]

On Wed, 21 Jan 2009, Sergey Goriatchev wrote:

> Dear Gabor,
>
> Thanks for that!
> Still, it is not really similar to how EDATE works.
> With julian(Sys.Date(), Sys.Date() - 10)  one moves 10 days back.
> The problem is that I need to move by months, not by days, as months
> have different number of days.
> I need to come to the same day when I move backward or forward in
> time, for example going back one month from today (21.01.2009) I need
> to come to 21.12.2008.
> I've read through your article in RNews 4/1 but still do not know how
> to do what I need to do.

The trick is to use the POSIXlt class.  E.g.

> x <- Sys.Date()
> xx <- as.POSIXlt(x)
> xx$mon <- xx$mon - 6
> as.Date(xx)
[1] "2008-07-21"

Now, the issue is what date is 6 months before 2008-08-31, and I'll 
leave you to ponder what that means.

>
> Regards,
> Sergey
>
> On Wed, Jan 21, 2009 at 10:44, Gabor Grothendieck
> <ggrothendieck at gmail.com> wrote:
>> See ?julian
>>
>>> julian(Sys.Date(), Sys.Date() - 10) # 10
>> [1] 10
>> attr(,"origin")
>> [1] "2009-01-11"
>>
>> and R News 4/1.
>>
>> On Wed, Jan 21, 2009 at 4:37 AM, Sergey Goriatchev <sergeyg at gmail.com> wrote:
>>> Hello, everyone
>>>
>>> I wonder if R has something similar to Excel function
>>> EDATE(start_date; months) which returns a serial number of the date
>>> that is the indicated number of months before of after the start date.
>>> Example (the second column EDATE(first_column; -6)):
>>> 01.01.1999      01.07.1998
>>> 02.02.1999      02.08.1998
>>> 06.03.1999      06.09.1998
>>>
>>> I am working with a zoo object where the row names are dates and for
>>> particular rows I need to find values that were recorded 6 months
>>> before (or return NA if the date is before the timeseries start).
>>>
>>> Maybe someone knows a passable R function for that kind of operation?
>>>
>>> Thanks in advance for help!
>>>
>>> Best,
>>> Sergey
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>
>
>
> --
> I'm not young enough to know everything. /Oscar Wilde
> Experience is one thing you can't get for nothing. /Oscar Wilde
> When you are finished changing, you're finished. /Benjamin Franklin
> Tell me and I forget, teach me and I remember, involve me and I learn.
> /Benjamin Franklin
> Luck is where preparation meets opportunity. /George Patten
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595