A few months ago I worked with a data set that had three separate variables to capture a date: Year, month, day. This isn’t very practical, and I was looking into a way to combine these in R. It’s not hard, all that’s needed is the paste and as.Date functions:
as.Date(paste(year, month, day, sep='-'))
paste combines the variables, and I specify the separator to be simple dashes, giving dates in the form “2013-11-24” (apparently ISO 8601). That’s the first form the function checks; no need to complicate things here. as.Date turns this into a Date object so that the variable behaves as expected. Yes, there is a capital “D” here.
Didier Ruedin 