[R] difference between 2 dates: IN MONTHS the way Motherscompute it

[Francisco J. Zagmutt]

Thanks Gabor, that's what I understood but then why do I get these results?

library(zoo)

12*as.numeric(as.yearmon("2006-03-07")-as.yearmon("2006-02-07"))
[1] NA
Warning messages:
1: NAs introduced by coercion
2: NAs introduced by coercion

12*as.numeric(as.yearmon(2006-03-07)-as.yearmon(2006-02-07))
[1] -12

12*as.numeric(as.yearmon(as.Date("2006-03-07"))-as.yearmon(as.Date("2006-02-07")))
[1] 1

version
         _
platform i386-pc-mingw32
arch     i386
os       mingw32
system   i386, mingw32
status
major    2
minor    2.1
year     2005
month    12
day      20
svn rev  36812
language R

I am sure I am missing somehting, but what is it??

Thanks

Francisco


>From: "Gabor Grothendieck" <ggrothendieck at gmail.com>
>To: "Francisco J. Zagmutt" <gerifalte28 at hotmail.com>
>CC: pzs6 at cdc.gov, r-help at stat.math.ethz.ch
>Subject: Re: [R] difference between 2 dates: IN MONTHS the way 
>Motherscompute it
>Date: Fri, 10 Mar 2006 21:01:55 -0500
>
>as.yearmon has a character method so its not necessary to
>convert it to Date first:
>
> > print(as.yearmon.character)
>function (x, ...)
>as.yearmon(as.Date(x, ...))
>
>On 3/10/06, Francisco J. Zagmutt <gerifalte28 at hotmail.com> wrote:
> > Gabor, please correct me if I am wrong but shouldn't you use as.Date to
> > change the date string to a "Date" class before you call as.yearmon?  
>i.e.
> >
> > 12*
> > 
>as.numeric(as.yearmon(as.Date("2006-03-07"))-as.yearmon(as.Date("2006-02-07")))
> >
> > That returns 1 in Windows XP
> >
> > Regards
> >
> > Francisco
> >
> >
> >
> > >From: "Gabor Grothendieck" <ggrothendieck at gmail.com>
> > >To: "Smith, Phil" <pzs6 at cdc.gov>
> > >CC: r-help at stat.math.ethz.ch
> > >Subject: Re: [R] difference between 2 dates: IN MONTHS the way
> > >Motherscompute it
> > >Date: Fri, 10 Mar 2006 13:38:12 -0500
> > >
> > >zoo has the "yearmon" class which will convert a date to a year +
> > >0/12 to 11/12 to represent the month, disregarding the day, so:
> > >
> > >library(zoo)
> > >12 * as.numeric((as.yearmon("2006-03-07") - as.yearmon("2006-02-06"))) 
># 1
> > >
> > >On 3/10/06, Smith, Phil <pzs6 at cdc.gov> wrote:
> > > > Hi R-people:
> > > >
> > > > I need a function to compute the number of months between 2 dates, 
>in
> > >the same way a mother would do it.
> > > >
> > > > For example, if a kid is born on February 6, the number of months
> > >between that date and March 7 is exactly 1 month, although it is only 
>29
> > >days.
> > > >
> > > > Thank you!
> > > > Phil Smith
> > > > CDC
> > > >
> > > >
> > > >
> > > >
> > > >        [[alternative HTML version deleted]]
> > > >
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