(* Taken from https://github.com/gopiandcode/ocamlot *)
let parse_date = Parser.parse_date
let parse_date_exn = Parser.parse_date_exn

let to_utc_string t =
  let www =
    Ptime.weekday t |> function
    | `Sat -> "Sat"
    | `Fri -> "Fri"
    | `Mon -> "Mon"
    | `Wed -> "Wed"
    | `Sun -> "Sun"
    | `Tue -> "Tue"
    | `Thu -> "Thu"
  in
  let (yyyy, mmm, dd), ((hh, mm, ss), _) = Ptime.to_date_time t in
  let mmm =
    match mmm with
    | 1 -> "Jan"
    | 2 -> "Feb"
    | 3 -> "Mar"
    | 4 -> "Apr"
    | 5 -> "May"
    | 6 -> "Jun"
    | 7 -> "Jul"
    | 8 -> "Aug"
    | 9 -> "Sep"
    | 10 -> "Oct"
    | 11 -> "Nov"
    | 12 -> "Dec"
    | _ -> assert false
  in
  Printf.sprintf "%s, %02d %s %04d %02d:%02d:%02d GMT" www dd mmm yyyy hh mm ss