[R] Undesired result

[Duncan Murdoch]

On 17/02/2021 9:50 a.m., Val wrote:
> HI All,
> 
> I am reading a data file which has different date formats. I wanted to
> standardize to one format and used  a library anytime but got
> undesired results as shown below. It gave me year 2093 instead of 1993
> 
> 
> library(anytime)
> DFX<-read.table(text="name ddate
>    A  19-10-02
>    D  11/19/2006
>    F  9/9/2011
>    G1  12/29/2010
>    AA   10/18/93 ",header=TRUE)
>      getFormats()
>      addFormats(c("%d-%m-%y"))
>      addFormats(c("%m-%d-%y"))
>      addFormats(c("%Y/%d/%m"))
>      addFormats(c("%m/%d/%y"))
> 
> DFX$anew=anydate(DFX$ddate)
> 
> Output
>   name      ddate       anew
> 1    A   19-10-02 2002-10-19
> 2    D 11/19/2006 2020-11-19
> 3    F   9/9/2011 2011-09-09
> 4   G1 12/29/2010 2020-12-29
> 5   AA   10/18/93 2093-10-18
> 
> The problem is in the last row. It should be  1993-10-18 instead of 2093-10-18
> 
> How do I correct this?

This looks a little tricky.  The basic idea is that the %y format has to 
guess at the century, but the guess depends on things specific to your 
system.  So what would be nice is to say "two digit dates should be 
assumed to fall between 1922 and 2021", but there's no way to do that 
directly.

What you could do is recognize when you have a two digit year, and then 
force the result into the range you want.  Here's a function that does 
that, but it's not really tested much at all, so be careful if you use 
it.  (One thing:  I recommend the 'useR = TRUE' option to anydate(); it 
worked better in my tests than the default.)

adjustCentury <- function(inputString,
                           outputDate = anydate(inputString, useR = TRUE),
                           start = "1922-01-01") {

   start <- as.Date(start)

   twodigityear <- !grepl("[[:digit:]]{4}", inputString)

   while (length(bad <- which(twodigityear & outputDate < start))) {
     for (i in bad) {
       longdate <- as.POSIXlt(outputDate[i])
       longdate$year <- longdate$year + 100
       outputDate[i] <- as.Date(longdate)
     }
   }
   longdate <- as.POSIXlt(start)
   longdate$year <- longdate$year + 100
   finish <- as.Date(longdate)

   while (length(bad <- which(twodigityear & outputDate >= finish))) {
     for (i in bad) {
       longdate <- as.POSIXlt(outputDate[i])
       longdate$year <- longdate$year - 100
       outputDate[i] <- as.Date(longdate)
     }
   }
   outputDate
}

library(anytime)
DFX<-read.table(text="name ddate
   A  19-10-02
   D  11/19/2006
   F  9/9/2011
   G1  12/29/2010
   AA   10/18/93
   BB   10/18/1893
   CC   10/18/2093",header=TRUE)

addFormats(c("%d-%m-%y"))
addFormats(c("%m-%d-%y"))
addFormats(c("%Y/%d/%m"))
addFormats(c("%m/%d/%y"))

DFX$anew=adjustCentury(DFX$ddate, start = "1921-01-01")
DFX
#>   name      ddate       anew
#> 1    A   19-10-02 2019-10-02
#> 2    D 11/19/2006 2006-11-19
#> 3    F   9/9/2011 2011-09-09
#> 4   G1 12/29/2010 2010-12-29
#> 5   AA   10/18/93 1993-10-18
#> 6   BB 10/18/1893 1893-10-18
#> 7   CC 10/18/2093 2093-10-18