[R] adding 1 month to a date

[Jim Porzak]

 OTOH,

 > seq(as.Date("2004-01-31"), by = "month", length = 14)
  [1] "2004-01-31" "2004-03-02" "2004-03-31" "2004-05-01" "2004-05-31"
  [6] "2004-07-01" "2004-07-31" "2004-08-31" "2004-10-01" "2004-10-31"
 [11] "2004-12-01" "2004-12-31" "2005-01-31" "2005-03-03"

 I would prefer to see dates forced to be within each month, not
 "leaking" into next month.

 IOW:
  [1] "2004-01-31" "2004-02-29" "2004-03-31" "2004-04-30" "2004-05-31", etc


 --
 Jim Porzak
 Loyalty Matrix Inc.
 San Francisco, CA


> On 10/12/05, Marc Schwartz <MSchwartz at mn.rr.com> wrote:
> > Thanks to Prof. Ripley for pointing this out.
> >
> > One of the approaches that I had considered here was to set up a vector
> > of the number of days in each month (adjusting of course for leap
> > years), and use "day arithmetic" to add/subtract the appropriate number
> > of days.
> >
> > However, it was easier to use seq.Date() and to further consider putting
> > a wrapper around it to make it yet even easier to use.
> >
> > Marc
> >
> > On Wed, 2005-10-12 at 13:23 +0100, Prof Brian Ripley wrote:
> > > On Wed, 12 Oct 2005, bogdan romocea wrote:
> > >
> > > > Simple addition and subtraction works as well:
> > > >  as.Date("1995/12/01",format="%Y/%m/%d") + 30
> > > > If you have datetime values you can use
> > > >  strptime("1995-12-01 08:00:00",format="%Y-%m-%d %H:%M:%S") + 30*24*3600
> > > > where 30*24*3600 = 30 days expressed in seconds.
> > >
> > > Sorry, not in general, as a month is not generally of 30 days (including
> > > in your example).
> > >
> > > seq.Date is a good way to do this.
> > >
> > > >
> > > >
> > > >> -----Original Message-----
> > > >> From: Marc Schwartz [mailto:MSchwartz at mn.rr.com]
> > > >> Sent: Tuesday, October 11, 2005 10:16 PM
> > > >> To: t c
> > > >> Cc: r-help at stat.math.ethz.ch
> > > >> Subject: Re: [R] adding 1 month to a date
> > > >>
> > > >>
> > > >> On Tue, 2005-10-11 at 16:26 -0700, t c wrote:
> > > >>> Within an R dataset, I have a date field called date_.
> > > >> (The dates are
> > > >>> in the format YYYY-MM-DD, e.g. 1995-12-01.)
> > > >>
> > > >>> How can I add or subtract 1 month from this date, to get
> > > >> 1996-01-01 or
> > > >>> 1995-11-01.
> > > >>
> > > >> There might be an easier way to do this, but using seq.Date(), you can
> > > >> increment or decrement from a Time 0 by months:
> > > >>
> > > >> Add 1 month:
> > > >>
> > > >> This takes your Time 0, generates a 2 element sequence (which begins
> > > >> with Time 0) and then takes the second element:
> > > >>
> > > >>> seq(as.Date("1995-12-01"), by = "month", length = 2)[2]
> > > >> [1] "1996-01-01"
> > > >>
> > > >>
> > > >>
> > > >> Subtract 1 month:
> > > >>
> > > >> Same as above, but we use 'by = "-1 month"' and take the
> > > >> second element:
> > > >>
> > > >>> seq(as.Date("1995-12-01"), by = "-1 month", length = 2)[2]
> > > >> [1] "1995-11-01"
> > > >>
> > > >>
> > > >> See ?as.Date and ?seq.Date for more information. The former
> > > >> function is
> > > >> used to convert from a character vector to a Date class object. Note
> > > >> that in your case, the date format is consistent with the default. Pay
> > > >> attention to the 'format' argument in as.Date() if your dates
> > > >> should be
> > > >> in other formats.
> > > >>
> > > >> HTH,
> > > >>
> > > >> Marc Schwartz
> >
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