adding 1 month to a date

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