Vectorized version

Get the first date of the month by cumulative month number.R

first_date_by_cmonth <- function(cmonth, base_year = 2017) {
  
  require(magrittr)
  require(dplyr)
  require(purrr)
  
  data.frame(
     index = seq(1, length(cmonth)),
     year = floor((cmonth - 1) / 12) + base_year,
     month = cmonth %% 12,
     day = rep(1, length(cmonth))
  ) %>% 
    mutate(month = case_when(month == 0 ~ 12, TRUE ~ month)) %>% 
    split(.$index) %>% 
    map_chr(function(df) {
      paste(df$year, df$month, df$day, sep = '/')
    }) %>% 
    unname() %>% 
    as.Date()
}