# Re: [R] adding 1 month to a date

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Wed 12 Oct 2005 - 22:23:10 EST

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@mn.rr.com]
>> Sent: Tuesday, October 11, 2005 10:16 PM
>> To: t c
>> Cc: r-help@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:
>>
>>
>> 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"
>>
>>
>> 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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>
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```--
Brian D. Ripley,                  ripley@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)
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