Use the package chron. Before importing the data to R from the cvs file, 
convert dates to numeric format. Dates are just a sequence from a starting 
point. I use the following to work with dates. Asuming you have a column in 
your cvs file with header "date":
 options(chron.origin=c(month=12,day=31,year=1899))
 x <- read.csv("../bla/bla.csv")
x$date <- chron(x$date,format="y-m-d")
 Or if you have your cvs file with labels like "02/03/2005", then replace 
the last line with:
 x$date <- chron(as.character(x$date),format="y-m-d")
 Then you can use your field date to do date operations
 Hope this is useful.

 On 7/27/05, John Sorkin <jsorkin@grecc.umaryland.edu> wrote: 
> 
> I am using read.csv to read a CSV file (produced by saving an Excel file
> as a CSV file). The columns containing dates are being read as factors.
> Because of this, I can not compute follow-up time, i.e.
> Followup<-postDate-preDate. I would appreciate any suggestion that would
> help me read the dates as dates and thus allow me to calculate follow-up
> time.
> Thanks
> John
> 
> John Sorkin M.D., Ph.D.
> Chief, Biostatistics and Informatics
> Baltimore VA Medical Center GRECC and
> University of Maryland School of Medicine Claude Pepper OAIC
> 
> University of Maryland School of Medicine
> Division of Gerontology
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> 
> 410-605-7119
> -- NOTE NEW EMAIL ADDRESS:
> jsorkin@grecc.umaryland.edu
> 
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