date import, incorrect century

Question

I have a bunch of dates that I am parsing that are in the form "%m/%d/%y". as.Date(dates, format = "%m/%d/%y") converts a date like "1/01/64" to "2064-01-01" but I need that to be "1964-01-01." I suppose I can find instances where the year is in the future and then subtract a century, but that seems a little ridiculous.

Answer 1

Dates are stored internal as integer days, so there is only such formatting at the time of input or output. As for input without century information I think you are out of luck. Here's what ?strptime says about the %y format spec: "On input, values 00 to 68 are prefixed by 20 and 69 to 99 by 19 – that is the behaviour specified by the 2004 and 2008 POSIX standards, but they do also say ‘it is expected that in a future version the default century inferred from a 2-digit year will change’."

  as.Date( "01/01/64", "%m/%d/%y", origin="1970-01-01") -100*365.25
  #[1] "1964-01-01"

It might be possible to start a bar fight about programmers who allow removal of century information given that Y2K is so recent in the past.

Since the default is to assume year 00-68 is 2000-2068, it is certainly possible to create an as.Dateshift

Answer 2

Another way to fix the dates is to change all years that occur in the future (relative to today's date using Sys.Date()) as starting with 19 instead of 20.

dates=as.Date(c("01/01/64", "12/31/15"))
# [1] "2064-01-01" "2015-12-31" ## contains an incorrect date

## Now correct the dates that havn't yet occurred
as.Date(ifelse(dates > Sys.Date(), format(dates, "19%y-%m-%d"), format(dates)))
#[1] "1964-01-01" "2015-12-31"