[R] Remove 100 years from a date object

[Barry Rowlingson]

On Fri, Dec 10, 2010 at 3:27 PM, Daniel Brewer <daniel.brewer at icr.ac.uk> wrote:
> Hello,
>
> I have some data that has dates in the form 27.02.37.  I convert them to
> a date object as follows:
> as.Date(data$date,format="%d.%m.%y")
>
> But this gives me years such as 2037 when I would like them to be 1937.
>  I thought of trying to take off some time i.e.
> as.Date(camCD$DoB,format="%d.%m.%y") - 100*365
> But that doesn't seem to work out correctly.  Any ideas how to do this?

Normally to adjust dates you can use as.difftime() and do arithmetic,
but a year is a variable thing (can be 365 or 366 days) so you cant
make a difftime of years. Days are variable things if you worry about
leap seconds...

Also, you could end up with an invalid date if you have 29-Feb-2000
and 29-Feb-1900. One wasn't a leap year...

A solution minus those caveats is to convert to POSIXlt and adjust the
$year element:

 > dob="27.02.37"
 > as.Date(dob,format="%d.%m.%y")
[1] "2037-02-27"
 > dobp = as.POSIXlt(as.Date(dob,format="%d.%m.%y"))
 > dobp$year = dobp$year - 100

 > dobp
[1] "1937-02-27 UTC"
 > as.Date(dobp)
[1] "1937-02-27"

although it might be easier to paste a '19' into your character variable

 > paste(substr(dob,1,6),"19",substr(dob,7,9),sep="")
[1] "27.02.1937"

and then do it the way you started. Assumes you have leading zeroes on
all fields though.

Barry