Re: finding differences in dates

[David L Lorenz <lorenz@usgs.gov>]

Walt,
  There was a thread on this topic
back in 2004. As you note, the computation of difference in years is complicated
by the differing number of days in years. A day is a fixed period of time,
so it makes sense to report the values of subtracting timeDate data in
days.
  A function to compute the age
of an individual was provided in that thread. I think that Tony Plate wrote
it. Here it is.

cal.years <- function(from, to) {
     caly <- numeric(length(from))
     for (i in seq(along=from))
{
         ay.rnd
<- round(as.numeric(to[i] - from[i]) / 365.25)
         target
<- from[i]+timeRelative(by="years", k.by=ay.rnd)
         targetp1
<- from[i]+timeRelative(by="years", k.by=ay.rnd+1)
         caly[i]
<- if (to[i]<target) ay.rnd-1 else if (to[i]>=targetp1) ay.rnd+1
else ay.rnd
     }
     return(caly)
}

  Note that this function will
give a value of 0 for the time span from 03/01/2006 to 02/28/2007 because
1 year from 03/01/2006 is 03/01/2007. You could simply add one to the result
to get the answer you want, or modify the code for your purpose. I have
not verified that the code works, but it did work for the example that
was used in the posting.
Dave

"Data Analytics Corp." <dataanalytics@earthlink.net>
Sent by: s-news-owner@lists.biostat.wustl.edu

10/05/2007 09:05 AM

To	s-news@lists.biostat.wustl.edu
cc
Subject	[S] finding differences in dates

Good morning,

I have a data frame with an anniversary and expiration date for people
belonging to a club.  The dates are in mm/dd/yyyy format.  I
want to 
find the number of years from anniversary to expiration.  I used 
timeDate(expiration) - timeDate(anniversary) but it looks like I get the
number of days.  Short of dividing by 365 (which won't allow for leap
years),  I want a function to find the number of years, preferably
to 
the next largest year (so if one anniversary is 03/01/2006 and 
expiration is 02/28/2007, then I want years = 1).  How do I do this?
 
Any help is appreciated.

Thanks,

Walt Paczkowski
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