Date vs date
On Fri, 14 Sep 2007, hadley wickham wrote:
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.
Arguably, it should (Is this a difftime object? Which units?). I'd advise against numeric operation on difftime objects in general, because of the unspecified units. These are always "days" when working with Date objects, but with general time objects it is not predictable. So I'd recommend sqrt(as.numeric(temp, units="days")).
It fails by design. Using sqrt() on a measurement that has an arbitrary origin would not have been good design.
Why not just always use seconds for difftime objects? An attribute could control how it was formatted, but would be independent of the underlying representation.
Because of leapseconds and changes to/from DST (which require knowing the timezone and its transition times).
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595