DateClosed <- factor(c("7/30/2007","12/12/2007", "5/8/2009"))
as.Date(DateClosed, format="%m/%d/%Y")
Produces:
[1] "2007-07-30" "2007-12-12" "2009-05-08"
Notice the capital "Y" in the format
param. The lower case "y" is for 2 digit years, so as.Date
reads the first two digits of the year token ("20"), and then assumes that refers to just the last two digits of the year, and adds the current date's century (also "20"), so you end up with dates in 2020.