[Rd] Date vs date

From: Terry Therneau <therneau_at_mayo.edu>
Date: Fri, 14 Sep 2007 12:22:17 -0500 (CDT)

 I wrote the date package long ago, and it has been useful. In my current task of reunifying the R (Tom Lumley) and Splus (me) code trees for survival, I'm removing the explicit dependence on 'date' objects from the expected survival routines so that they better integrate. Comparison of 'date' to 'Date' has raised a couple of questions.  

  Clearly Date is more mature -- more options for conversion, better plotting, etc (a long list of etc). I see three things where date is better. Only the last of these really matters, and is the point on which I would like comment. (Well, actually I'd like to talk you all into a change, of course).   

  1. Since date uses 1/1/1960 as the base, and so does SAS, those of us who contantly pass files back and forth between those two packages have a slightly easier time.
  2. as.date(10) works, as.Date(10) does not. Sometimes I have done a manipluation that the date package does not understand, and I know that the result is still of the right type, but the package does not. However, this is fairly rare and I can work around it. (It mostly occurs in processing the rate tables for expected survival).
  3. temp <- as.Date('1990/1/1') - as.date('1953/2/5') sqrt(temp) Error in Math.difftime(temp3) : sqrtnot defined for "difftime" objects

  Minor bug: no space before the word 'not'   Major: this shouldn't fail.

People will do things with time intervals that you have not thought of. Fitting a growth curve that uses a square root, for instance. I firmly believe that the superior behavior in the face of something unexpected is to assume that the user knows what they are doing, and return a numeric.

   I recognize that "assume the user knows what they are doing" is an anathema to the more zealous OO types, but in designing a class I have found that they often know more than me!    

   4. Variation on #3 above    

> (as.Date('2007-9-14') - as.Date('1953-3-10')) / 365.25
  Time difference of 54.51335 days    

    No, I am not 54.5 days old. Both hair color and knee creaking most definitely proclaim otherwise, I am sorry to say. Time difference / number should be a number.     

   5. This is only amusing. Im not saying that as.Date should necessarily work, but the format is certainly not ambiguous. (Not standard, but not ambiguous). Not important to fix, not something that date does any better.

> as.Date('09Sep2007')

Error in fromchar(x) : character string is not in a standard unambiguous format  

           Terry Therneau

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