OCaml Unix iso datetime converting

unix.ml

module Unix = struct
  include Unix
  let tm_of_iso date =
    let make y m d h mi s = Unix.(
        {tm_sec=s;tm_min=mi;tm_hour=h;tm_mday=d;tm_mon=(m - 1);tm_year=(y - 1900);
         tm_wday=0;tm_yday=0;tm_isdst=false}) in
    Scanf.sscanf date "%u-%u-%uT%u:%u:%uZ" make

  let iso_of_tm tm =
    Unix.(Printf.sprintf "%u-%02u-%02uT%02u:%02u:%02uZ" (tm.tm_year + 1900) (tm.tm_mon + 1) tm.tm_mday
            tm.tm_hour tm.tm_min tm.tm_sec)

  let cmp_tm d1 d2 =
    let ts1, _ = mktime d1 in
    let ts2, _ = mktime d2 in
    compare ts1 ts2

  let tm_of_yojson = function
      `String s -> Result.Ok (tm_of_iso s)
    | _ -> Result.Error "Unknown json type for date time"
end