You can use Sys.time(). However, when you record the time difference in a table or a csv file, you cannot simply say end - start. Instead, you should define the unit:
f_name <- function (args*){
start <- Sys.time()
""" You codes here """
end <- Sys.time()
total_time <- as.numeric (end - start, units = "mins") # or secs ...
}
Then you can use total_time which has a proper format.