Assuming that the current date is required
the_files <- dir()
the_files
#> [1] "calc.csv" "data.csv" "slides.csv"
the_month <- lubridate::month(Sys.Date(), label = TRUE)
the_month
#> [1] Dec
#> 12 Levels: Jan < Feb < Mar < Apr < May < Jun < Jul < Aug < Sep < ... < Dec
the_year <- lubridate::year(Sys.Date())%%2000
suffix <- paste0("_",the_month,the_year)
l <- strsplit(the_files,"[.]")
new_names <- list()
for(i in seq_along(l)) new_names[i] = paste0(l[[i]][1],suffix,".",l[[i]][2])
new_names <- unlist(new_names)
new_names
#> [1] "calc_Dec21.csv" "data_Dec21.csv" "slides_Dec21.csv"
for(i in seq_along(the_files)) file.rename(the_files[i],new_names[i])
dir()
[1] "calc_Dec21.csv" "data_Dec21.csv" "slides_Dec21.csv"