[R] Extracting part of date variable

[John Kane]

Thank you Peter.

It was not my question but I was just about to start
the morning's work by searching Help and RSiteSearch()
for this exact question. 

--- Peter Dalgaard <P.Dalgaard at biostat.ku.dk> wrote:

> stat stat wrote:
> > Dear all,
> >    
> >   Suppose I have a date variable:
> >    
> >   c = "99/05/12"
> >    
> >   I want to extract the parts of this date like
> month number, year and day. I can do it in SPSS. Is
> it possible to do this in R as well?
> >    
> >   Rgd,
> >
> >   
> Yes. One way is to use substr(), e.g.:
> 
> > substr(c,1,2)
> [1] "99"
> > as.numeric(substr(c,1,2))
> [1] 99
> 
> This also nicely sidesteps the ambiguity issue: 1999
> or 1899? May or
> December? On the other hand, you'll get in trouble
> if leading zeros are
> sometimes absent (strsplit() or gsub() if you want
> to pursue that route
> further).
> 
> For a more principled approach, use the time and
> date handling tools.
> Assuming that you can live with the system defaults
> for 2-digit years,
> 
> > strptime(c,format="%y/%m/%d")
> [1] "1999-05-12"
> > strptime(c,format="%y/%m/%d")$year
> [1] 99
> > strptime(c,format="%y/%m/%d")$mon
> [1] 4
> > strptime(c,format="%y/%m/%d")$mday
> [1] 12
> 
> Beware the peculiarities of the entries defined by
> POSIX standard, see
> ?DateTimeClasses, and also:
> 
>      '%y' Year without century (00-99). If you use
> this on input, which
>           century you get is system-specific.  So
> don't!  Often values
>           up to 69 (or 68) are prefixed by 20 and
> 70(or 69) to 99 by
>           19.
> 
> (I'm at a bit of a loss as to fixing up two digit
> years once the damage
> has been done. Presumably, you can just diddle the
> year field, but I'm a
> bit uneasy about the fact that  2000 was a leap year
> and 1900 was not.)
> 
> 
> 
> -- 
>    O__  ---- Peter Dalgaard             Øster
> Farimagsgade 5, Entr.B
>   c/ /'_ --- Dept. of Biostatistics     PO Box 2099,
> 1014 Cph. K
>  (*) \(*) -- University of Copenhagen   Denmark     
>     Ph:  (+45) 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             
>     FAX: (+45) 35327907
> 
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