In addition to what's said already, I would just like to point out the very
useful R Help Desk article "Date and Time classes in R" in R News June 2004
by Gabor Grothendieck and Thomas Petzoldt.

>-----Original Message-----
>From: r-sig-finance-bounces@stat.math.ethz.ch 
>[mailto:r-sig-finance-bounces@stat.math.ethz.ch] On Behalf Of 
>Achim Zeileis
>Sent: 22 March 2006 02:19 PM
>To: P. Hénaff"
>Cc: r-sig-finance@stat.math.ethz.ch
>Subject: Re: [R-sig-finance] Date calculations
>
>On Wed, 22 Mar 2006 00:38:20 -0600 P. Hénaff wrote:
>
>> I'm a rank beginner in R and R-metrics, but experienced in 
>matlab and 
>> other languages. I'm strugling with date arithmetics in R. I'm doing 
>> statistical analysis on financial time series, using its, which uses 
>> POSIXct objects to measure time. What is the best way to perform 
>> simple date arithmetic with these objects? I do not care 
>about h/m/s, 
>> nor do I care about daylight saving time.
>
>In that case I would recommend to go with the "Date" class 
>available in base R, e.g.
>  as.Date("2005-04-03") + 0:10
>which should be rather self-explanatory.
>
>The package zoo provides time series infrastructure where the 
>time information can be stored in "Date" (or virtually any 
>other class).
>  library("zoo")
>  z <- zoo(rnorm(11), as.Date("2005-04-03") + 0:10)
>  plot(z)
>
>hth,
>Z
>
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