library(stringr)

format_to_date <- function(x){
  year <- str_extract(x, "^\d{4}")
  if (grepl("M", x)) {
    month <- str_pad(str_extract(x, "\d{1,2}$"), width = 2)
    paste0(year, "-", month, "-01")
  } else if (grepl("Q", x)) {
    month <- as.numeric(str_extract(x, "\d{1}$"))
    month <- month + (month - 1) * 3
    paste0(year, "-", month, "-01")
  } else{
    paste0(year, "-01-01")
  }
}

Dframe <-
  data.frame(string = c("2008M5", "2009Q3", "2011"),
             stringsAsFactors = FALSE)

as.Date(vapply(Dframe$string, format_to_date, character(1)))

df <- data.frame(Raw = c("2008M5", "2009Q3", "2011"))

convertDate <- function(x) {
    res <- data.frame(DFreq = "Annual",
                      Date = paste0("1/12/", x))
    if (length(grep("Q", x)) == 1) {
        foo <- unlist(strsplit(x, "Q"))
        res <- data.frame(DFreq = "Quarterly",
                          Date = paste0("1/", as.numeric(foo[2]) * 3, "/", foo[1]))
    } else if (length(grep("M", x)) == 1) {
        foo <- unlist(strsplit(x, "M"))
        res <- data.frame(DFreq = "Monthly",
                          Date = paste0("1/", foo[2], "/", foo[1]))
    }
    return(res)
}
res <- apply(df, 1, convertDate)
res <- do.call("rbind", res)

      DFreq      Date
1   Monthly  1/5/2008
2 Quarterly  1/9/2009
3    Annual 1/12/2011