[R] date/time arithmetic

[Gabor Grothendieck]

On 11/30/05, Parlamis Franklin <fparlamis at mac.com> wrote:
> I agree there are workarounds (although reduction to secs is not
> necessarily graceful when you want to add, say, "1 month" to a date,

You can do that without difftime via:

  seq(x, len = 2, by = "month")[2]

> or any other increment for which the total number of seconds is not
> fixed--and is also complicated by the existence of leap seconds).
> But upon inspection, they are never as clean as they seem.
>
> The representation in the man pages that difftime / date-time
> arithmetic was possible made me hopeful that some of these tasks had
> been handled already, in a manner consistent with the internal date-
> time functions.
>
>
> On Nov 30, 2005, at 9:42 AM, bogdan romocea wrote:
>
> > What do you need a bunch of functions for? I'm not familiar with the
> > details of difftime objects, however an easy way out of here is to get
> > the time difference in seconds, which you can then add or subtract as
> > you please from date-times.
> >
> > x<-Sys.time(); y<-Sys.time()+3600
> > diff <- as.numeric(difftime(y,x,units="secs"))
> > x; y; diff
> > y-diff
> >
> >
> >> -----Original Message-----
> >> From: r-help-bounces at stat.math.ethz.ch
> >> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of
> >> Parlamis Franklin
> >> Sent: Wednesday, November 30, 2005 1:57 PM
> >> To: Parlamis Franklin
> >> Cc: r-help at stat.math.ethz.ch
> >> Subject: Re: [R] date/time arithmetic
> >>
> >>
> >> Hi.  Sorry to pester, but I didn't get a reply to this
> >> (perhaps owing
> >> to Thanksgiving break).  This seems to be a case where the R
> >> language
> >> simply doesn't do what it says it does (i.e., allow addition or
> >> subtraction of a difftime object and a date-time object).
> >>
> >> Am I wrong?  I am about to write a bunch of functions (easy, but
> >> perhaps redundant) to do this, and I just want to know whether there
> >> is a bug here, or whether I am missing something obvious.
> >>
> >> Thanks.
> >>
> >> FP
> >>
> >> On Nov 23, 2005, at 10:02 AM, Parlamis Franklin wrote:
> >>
> >>> On the help page "DateTimeClasses {base}" it says:
> >>>
> >>> "One can add or subtract a number of seconds or a difftime object
> >>> from a date-time object, but not add two date-time objects."
> >>>
> >>> However,
> >>>
> >>>> x<-Sys.time(); y<-Sys.time()+3600
> >>>> diff<-y-x
> >>>> x; y; diff
> >>> [1] "2005-11-23 19:58:20 GMT"
> >>> [1] "2005-11-23 20:58:20 GMT"
> >>> Time difference of 1 hours
> >>>> y-diff
> >>> [1] "2005-11-23 20:58:19 GMT"
> >>> Warning message:
> >>> Incompatible methods ("-.POSIXt", "Ops.difftime") for "-"
> >>>
> >>> Do I have the syntax wrong?  Or is some conversion of the difftime
> >>> object to raw seconds necessary prior to performing arithmetic?
> >>> And is the help page wrong?
> >>>
> >>> Thanks.
> >>>
> >>> Franklin
> >>>
> >>>
> >>>
> >>>
> >>
> >>
> >>      [[alternative HTML version deleted]]
> >>
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