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

[Josip Dasovic]

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.

Thanks,
Josip

Research Associate
Human Security Report Project
School for International Studies
Simon Fraser University
Suite 7200--515 W. Hastings St.
Vancouver, BC V6B 5K3 Canada