let _ =
  let s = Num.num_of_string (Printf.sprintf "%.0f" (Unix.gettimeofday ())) in
  let ps = Num.mult_num (Num.num_of_string "1000000000000") s in
  Printf.printf "%s picoseconds have passed since January 1st, 1970.\n"
    (Num.string_of_num ps)
;;