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

[Gabor Grothendieck]

The first number is the year,
the second is the month and the third is the day.
It ignores trailing characters.

> library(chron)
> f <- function(x) str(month.day.year(as.Date(x)))

> f("2001/1/1")
List of 3
 $ month: num 1
 $ day  : num 1
 $ year : num 2001

> f("1/1/2001")
List of 3
 $ month: num 1
 $ day  : num 20
 $ year : num 1

> # trailing 01 ignored.
> f("13/1/2001")
List of 3
 $ month: num 1
 $ day  : num 20
 $ year : num 13

> # no 13th month
> as.Date("1/13/2001")
Error in fromchar(x) :
  character string is not in a standard unambiguous format

On Thu, Jan 22, 2009 at 1:54 PM, Marie Sivertsen <mariesivert at gmail.com> wrote:
> 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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