Format dates to Month-Year while keeping class Date

You could use the yearmon class from the zoo package

require("zoo")
ds <- data.frame(person_id=1:3, date_visit=c("2012-05-03", "2012-08-13", "2012-12-12"))
ds$date_visit <- as.yearmon(ds$date_visit)
ds
  person_id date_visit
1         1   May 2012
2         2   Aug 2012
3         3   Dec 2012