[R] Date Not Staying in Date Format

Gabor Grothendieck ggrothendieck at gmail.com
Tue Jan 31 03:01:41 CET 2006

Using this test data, say:

   dd <- Sys.Date() + 0:9
   fac <- gl(5,2)

Try this:

   do.call("c", tapply(dd, fac, max, simplify = FALSE))

or if a list result is ok then just

   tapply(dd, fac, max, simplify = FALSE)

Another possibility using the as.Date.numeric method in the zoo
package is:

  as.Date(tapply(dd, fac, max))

For more info on the internal representation of "Date" class objects see
the Help desk article in R News 4/1.

On 1/30/06, David Randel <davidr00 at hotmail.com> wrote:
> I have a column in a data frame that has a class of "Date" and a mode of
> "numeric".  When I:
> max(df$Date)
> My output stays in Date format, i.e. "2006-01-03".
> However, when I run the following statment:
> tapply(df$Date, df$SomeFactor, max)
> my output looks like this:  9129   9493   9861  10226  10591  10956  11320
> 11687  12052  12417
> The returned object is of mode "numeric" and class "array".  Each array
> element is of mode "numeric" and class "numeric".  I believe that this is
> the integer representation of my date.  I can't seem to convert it back to a
> date.
> How do I get these to be intrepreted as a date instead of a number?
> Thanks,
> ~Dave R.
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