[Rd] Date vs date

[Terry Therneau]

 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