You can use the lubridate package to convert a variety of character formats into Dates.
If your data frame encodes dates as Sep 2017 you can use the my() function ("my" stands for "month-year") to convert that into a Date value representing September 1, 2017.
df <- data.frame(date = c("Sep 2017", "Oct 2011", "Jun 1912"))
df$date <- lubridate::my(df$date)