[R] Confusion with Converting Factors to Dates using as.date

[Peter Dalgaard]

Josip Dasovic wrote:
> Dear R-Helpers:
> 
> I'm having a problem getting dates into the correct format. I have a
> data frame, which is based on a .csv file that I imported into R via
> read.table.
> 
> R has converted my date variables to factors; when I use the as.Date
> command, most of the values are converted "correctly" (and by this I
> guess I mean converted "as I wish them to be") but some have not
> been.
> 
> Here's what I have: str(pk.df)
> 
> 'data.frame':	206 obs. of  134 variables: $ uniqid         : int  010
> 015 120 130 210 245 320 330 415 ... $ st_date     : Factor w/ 154
> levels "01/01/48","01/01/51",..: 46 27 NA 12 118 NA 63 127 NA NA ... 
> ...
> 
> I then convert them to a date class using
> 
> st_date.new<-as.Date(st_date, "%m/%d/%y")
> 
> This _seems_ to work...
> 
> str(st_date.new) Class 'Date'  num [1:206]  8150  8466    NA 33982
> 10149 ...
> 
> But notice the 4th observation; I would like it to be 1963, not 2063.
> 
> 
> st_date.new[1:10] [1] "1992-04-25" "1993-03-07" NA
> "2063-01-15" "1997-10-15" [6] NA           "1991-05-31" "1994-11-20"
> NA           NA
> 
> st_date[1:10] [1] 04/25/92 03/07/93 <NA>     01/15/63 10/15/97 <NA>
> 05/31/91 [8] 11/20/94 <NA>     <NA> 154 Levels: 01/01/48 01/01/51
> 01/01/52 01/01/59 01/01/63 ... 12/31/96
> 
> 
> I thought that the problem might be that I was converting a factor,
> so I first converted the variable to a character type (although I
> understand that this is done automatically) and then to date class,
> but I still had the same problem. Does anybody know how I can solve
> this and why I am getting this behavior? One more tidbit: the
> earliest date for which the date conversion is "correct" is
> 1969-04-15, while the most recent date for which the century is
> "incorrect" is 1967-11-05.

Well, to quote ?strptime:

      '%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 68 (or 69) are prefixed by 20 and 69 (or 70) to 99 by
           19.



-- 
    O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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