[R] Unexpected behaviour of the as.Date (was: Error as.Date on Invalid Dates)

[Greg Snow]

I believe the original thread was about whether the function returns NA or stops with an error when given an invalid date (such as Feb 29 in a non-leap year).  Your question was about how as.Date returned something different from what you expected.  Related, but different enough that it probably would have been better to start a new thread.

For your question, the help page for as.Date includes:

  "format: A character string.  The default is '"%Y-%m-%d"'.  For
          details see 'strftime'."

And 

"Character strings are processed as far as
     necessary for the format specified: any trailing characters are
     ignored."

I don't see anything in your examples that runs counter to the above.

Remember that computers do exactly what you tell them to do, not what you think that they should do.

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Marie Sivertsen
> Sent: Thursday, January 22, 2009 11:55 AM
> To: Brian D Ripley; R-help at stat.math.ethz.ch
> Subject: [R] Unexpected behaviour of the as.Date (was: Error as.Date on
> Invalid Dates)
> 
> Dear Brian,
> 
> I dont understand what you mean.  The thread was about the as.Date
> which you
> suggested to be used instead of the as.date.  Following your advice I
> tried
> the as.Date and have questions about the observed behaviour, which was
> surprising to me.  Is this what you call hijacking?  Do you mean I
> ought
> start a new thread instead?  I thought my question were relevant to the
> threads' subject.  I am sorry if it were not.
> 
> So here is the questions once again:  why do the as.Date behave as in
> my
> examples below, is this intended?
> 
> 
> 
> On Thu, Jan 22, 2009 at 3:55 PM, Brian D Ripley
> <ripley at stats.ox.ac.uk>wrote:
> 
> > You've hijacked a thread here.
> >
> >
> > On Thu, 22 Jan 2009, Marie Sivertsen wrote:
> >
> >
> >> I am relatively new to R, so maybe I am miss something, but I now
> tried
> >> the a
> >> s.Date now and have problems understanding how it works (or don't
> work as
> >> it seem).
> >>
> >> Brian D Ripley wrote:
> >> > On Thu, 22 Jan 2009, Terry Therneau wrote:
> >> >>
> >> >> One idea is to use the as.date function, for the older (and less
> >> capable) 'date'
> >> >> class.  This is currently loaded by default with
> library(survival).  It
> >> re
> >> turns
> >>
> >> >> NA for an invalid date rather than dying.
> >> >
> >> > So does as.Date *if you specify the format* (as you have to with
> your
> >> as.da
> >> te:
> >> > it has a default one):
> >
> >
> 
> My examples:
> 
> 
> 
> >
> >> as.Date("2001/1/1")
> >> Works fine
> >> as.Date("1/1/2001")
> >> Prints "1-01-20" ???
> >> as.Date("13/1/2001")
> >> Prints "13-01-20" ???
> >>
> >> as.Date("1/13/2001")
> >> Prints error: not in standard unambigous format
> >> It seems that as if both "1/1/2001" and "13/1/2001" were considered
> by R
> >> to b
> >> e in a standard unambiguous format (or otherwise an error be
> reported?)
> >> and yet they
> >>
> >>
> >> are parsed incorrectly according to what one could think is obvious.
> It
> >> is a
> >> lso surprizing that not only "13/1/2001" but also "1/2/2001" and
> >> "2/1/2001" are successful but incorrect parsed as if they are
> unambiguous,
> >> and yet
> >> "13/1/2001" is ambiguous, though there is really just one way to
> parse it
> >> meaningfully.
> >> I think the strings that are incorrectly parsed should raise errors,
> and
> >> the last example should be succesful parsed.  What is the reason for
> the
> >> observed
> >> ?
> >>
> >>
> >>
> 
> 
> Mvh.
> Marie
> 
> 
> 
> >
> >>
> > --
> > Brian D. Ripley,                  ripley at stats.ox.ac.uk
> > Professor of Applied Statistics,
> http://www.stats.ox.ac.uk/~ripley/<http://www.stats.ox.ac.uk/%7Eripley/
> >
> > University of Oxford,             Tel:  +44 1865 272861 (self)
> > 1 South Parks Road,                     +44 1865 272866 (PA)
> > Oxford OX1 3TG, UK                Fax:  +44 1865 272595
> >
> 
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